Rules

I'm inclined to think of a standard healing potion as adventuring gear, but only because it appears in the adventuring gear list. I don't have a strong opinion one way or the other, although...

My eventual plan is for Aanbo to take the Way of Mercy, which at third level allows him to spend ki points to heal HP. If the consensus is that healing potions are magical and require the Cure Wounds spell, could Aanbo's Hand of Healing feature do the same?

Adventuring gear.

I don't have a strong feeling either way, but I'll go with adventuring gear.

With the casting of Spirit Guardians, the question has arisen: when does a persistent area of effect spell activate/take effect?

A spell that creates a momentary area of effect - Shatter, Fireball, et cetera - will do damage immediately.

But a spell that creates a persistent area of effect - Blade Barrier, Cloud of Daggers, Healing Spirit, Ravenous Void, Spiritual Guardians, et cetera - is described as taking effect when either:
a) a character first moves into that area on a turn
or
b) a character starts its turn in the area

It turns out that there is a LOT of debate about what that means. I'm using this post to describe my ruling on it: when it applies, and when it doesn't.

There are five possible scenarios/cases that could be interpreted as applicable.
1) A character is inside the area of effect when the spell is cast.
I've decided that this does NOT apply. Yes, the character is in the area of the spell, but the character did not move to get there. Since the spell's area of effect appeared around the character, rather than the character moving into the spell's area of effect, it does not apply.
Wizard Jo casts the spell, and Loren is in the area of effect, but Loren does not take damage when the spell was cast on Wizard Jo's turn.

2) The spell has already been cast, and a character starts its turn in the area of the spell.
Obviously this applies. So if Wizard Jo casts Cloud of Daggers and Loren is in the area of effect at the start of Loren's turn, then Loren is affected by the spell (with Cloud of Daggers, Loren takes 4d4 piercing damage at the start of their turn).

3) The spell has already been cast, and the caster moves the area of effect to include a character.
This does NOT apply, either, because the character being affected is not the one moving. Ranger Pat casts Healing Spirit then moves the Healing Spirit into Loren's space. Loren doesn't get healed then; Loren can be healed at the start of Loren's turn if the Healing Spirit is still there.

4) The spell has already been cast, and a character outside of the spell moves into the area of effect on their turn.
This applies. Wizard Jo has cast Evard's Black Tentacles, and then on Loren's turn, Loren enters the area of that spell. Loren then has to immediately roll the DEX saving throw or take 3d6 bludgeoning damage and be restrained.

5) The spell has already been cast, and a character inside the spell's area of effect is moved out of the area of effect on someone else's turn.
This does NOT apply. Wizard Jo has cast Moonbeam, and Loren is caught in the area of effect. Wizard Jo ends their turn. A third character, Druid Jules, takes their turn, casting Thorn Whip on Loren. Loren takes damage, fails their saving throw, and is pulled out of Moonbeam's area of effect. Loren is moving, but not INTO the area of effect. Whether or not Loren is moving OUT of the area of effect, they are already in the area of effect, rather than moving into it.

6) The spell has already been cast, and a character outside the spell's area of effect is moved into the area of effect on someone else's turn.
This applies. Wizard Jo casts Cloudkill and ends their turn, with Loren safely 5' outside the cloud. Druid Jules casts Thunderwave, Loren fails their save, and Loren takes damage from the Thunderwave...AND gets pushed backwards into the Cloudkill area of effect. Loren immediately has to roll the CON saving throw. Cloudkill affects Loren because they moved INTO the area of effect.

FYI, just to set expectations: Orlane and Hochoch are about 50 miles apart. On foot, there's just over two days' full travel between Hochoch and Orlane. Up to this point, I haven't been tracking pace or asking you to roll Exhaustion checks to cover the "remainder" distance. However, now we're talking about PCs heading in opposite directions and trying to meet up at a specified time, so I'm going to have to track it more closely.

Per the 5e travel rules, each normal day of travel involves 8 hours of active hiking (which covers 24 miles on a road*), but I've been narrating that this normal pace has been interspersed with rests, calls of nature, meals, and so on, so that the party traveling for a day takes up the daylight hours (~12 hours). The party is currently a bit to the east of the midway point between Hochoch and Orlane, a couple hours closer to Orlane. Heading west at a normal pace from where the party is now, PCs will arrive at Hochoch a bit after nightfall (ie, in about 14 hours at normal pace, because I'm not having you roll exhaustion checks for the last couple of miles but it does take a bit more time). How much after nightfall depends on whether they make detours to retrieve undead-dropped items (at the previous campsite) and/or visit the Jarls (which is only a little bit further than that, since the Jarls live about a full day's normal pace walk outside of Hochoch). They could choose to move at a fast pace, and arrive right around nightfall (~3 hours faster). You can also tell me that you're going to push yourselves to hike longer than the normal day of travel, which would allow you to cover another 2-4 miles per hour (depending on pace) at the cost of everyone needing to make Exhaustion CON saves every hour.

Regardless of the pace, every hour that Chase heads east adds an hour to the time he will need to make it back to Hochoch at that same pace. So if the goal is to meet in ~24 hours from now, he can take two hours to explore the woods to the east (taking him 6 miles, so he'd be 36 miles from Hochoch), and then he'd need to start heading back west, and he would be hiking all night to get to Hochoch at a fast pace (and the last two hours he'd need to make Exhaustion checks). If he wants to explore/track all the way back to Orlane (if the tracks lead there), he expects that that will take a five hours at a normal pace (which he would be doing, because he can move at a normal pace and still be stealthy because he's a ranger in his favored terrain). Short of stealing a flying mount or using some kind of transportation magic, it is going to be very difficult for Chase to then arrive in Hochoch 19 hours after that (a 50-mile distance), even if he returned at a fast pace rolling Exhaustion checks every hour after the first 12 miles (3 hours). Put another way: if Chase goes to Orlane today, he could be back in Hochoch safely in 3 days' time (early on the 3rd day at a normal pace, or really early on the 3rd day if he returns at a fast pace), or dangerously in as little as 18 hours of travel (5 hours to get to Orlane, then immediately turning back at a fast pace for 3 hours, taking no rests, and then rolling an exhaustion check 10 times to travel the remaining distance...which might literally kill Chase).

* - This ignores how amazingly fast this is, as experienced hikers in the real world will hike all day and travel about 16 miles at a fast pace (10-12 is much more normal for experienced hikers). Maintaining a 3 MPH pace for a full day while carrying all their possessions is just a sign of how incredibly fit PCs are.

Chase won't have to go all the way back to Orlane. He just wants to be sure where that group of cultists came from. If it is pretty much from the direction of Orlane that will be good enough for him. He really wants to see if he can find where there the undead are coming from. Id it is the direction or area.

Two rules clarifications.

First, initiative for companions: I'm not going to change it in the middle of this encounter, but from the end of this encounter onward, players of PCs with NPC companions (familiars, steel defenders, ranger beasts, etc) will be rolling initiative for those companions (when applicable, as for a familiar but not a steel defender).

Next, a clarification on Help (again, this will apply after this encounter): when a character is performing a Help action, the player must declare which character is receiving the Help, just as we'd do in a real-time game. For example, it should be "On my turn, I'm Helping Jenny" and then Jenny's player needs to apply that ADV on her next roll, not "I'm Helping the group!" with the players then debating who will "take" the Help or, even worse, deciding after they roll which skill check/attack will have ADV.

That's fair. I admit, I totally forgot about my familiar when the battle started. I just assumed that the lack of initiative would just mean the familiar acted after me.

As for the Help action, I think it's a bit difficult to predict who I would assist because we aren't posting in initiative order, so for example it's tough to say that I would help Dell when I submit my post first, but he acts before me and decides to attack a different target. If we were at a real-time table, it's easy to ask, receive an answer, and immediately adjust and decide what to do on my turn. It may not become a wasted action because Dell may use that assistance in his next turn, but in PbP that requires all of us to remember what I did the previous turn.

I will do my best, of course. I think I need to pay closer attention to the actual initiative order. I'm just a bit concerned that I'll be stuck waiting for people to actually post their actions if they go before me, or worse by declaring who I help, I'm now passively forcing that person after me to do something they may not want to.

I could also be overthinking it. I'll probably have a better feel for how to perform the Help action as specified in the PHB within this PbP format after we get a few more rounds in.

Thanks for the clarification, I’ll be rolling initiatives for Berthold’s retinue moving forward.

Note: I've edited this post based on this clarification/correction.
This is the tension that exists in a real-time game, too. Note though that you (the person Helping) do not have to describe HOW you are helping, nor do you have to specify which action you are Helping. Mechanically, you just say "I'm Helping that character" and then it is on them (the person whose character was specified) to choose the first attack/check their character performs (which is the action that benefits from your Help). Or you can Help an attack, by specifying the target enemy adjacent to the Helper. The next attack on that target (before the Helper's next turn) is made with ADV, regardless of who is making that attack. So in your example, you'd declare that Grallaria is Helping Dell, and it doesn't matter which target Dell attacks: his to attack a particular target, and the next attack benefits from the Help. Even if he chose to perform a non-attack action, the associated skill check would benefit from the Help.

I know that for Play-by-Post, where we aspire to describe everything that our characters are doing, it is counter-intuitive to NOT describe how we are Helping, but that is literally how the mechanic works RAW. In practice, in real-time games I've run/played, the Helping player usually interjects after the Helped player decides on the action, providing a sentence on what form their assistance takes, but but I can't really think of a way that would be practicable but not prescriptive in PbP. Unless we can figure out a way to do that, we can just go with RAW: declare who you're Helping/Helping to attack, and thatthe affected character's player then rolls the next check/attack with ADV.

While I understand that I can just declare the Help action for another person to provide advantage on their ability check, it's different for combat. The one Helping needs to be within 5 feet of the target in order to grant advantage on their ally's first attack roll. At least, that's been my interpretation, which is why I had the owl fly next to H4.

Yes, then I'll amend what I wrote previously. Per your example:
It's round one. You want to help Dell attack a target, but his initiative is higher than the Helper's initiative. Regardless of when his player posts, his turn will come before the Helper's, so you cannot Help him in round one. You would declare a target within 5' of the Helping character. The first ally to attack that target will roll with ADV on their first attack on that target. So your Help would take effect for Dell if:
a) he chooses to attack that target on his turn in round two,
AND
b) he's the first ally to attack the target after the Helper's turn.
But, contrary to what I wrote earlier, it doesn't need to be Dell that makes the attack. Any ally can get the benefit of that attack Help, as long as they are the first one to attack the target you've specified after you've performed the Help action. That could happen in round one, or it could happen in round two, as long as the attack happens after the Help action and before the Helper's next turn.

I'm hoping this allays your concern about "a wasted action" because, for the Help to be ineffective, it would mean that no one attacks the specified target for an entire round.

That makes more sense to me when helping to target an attack, with the advantage going to the first attacker after the help action.

*Help action distracts granting advantage*

Just a reminder for everyone: stabilizing a dying creature is automatic with one application of a Healer's Kit.
A medicine check is needed only for anyone trying to Stabilize a character WITHOUT a Healer's Kit.

Which means that, in future, anyone can try to Stabilize a dying companion; you never need to wait for a Healer's Kit to Stabilize. That's basic RAW.

The variant rule involving Healer's Kits that we're using in this game is that natural healing (ie, healing with Hit Dice) requires using a Healer's Kit. So it's hit dice healing, not stabilizing, that needs a Healer's Kit (though a Healer's Kit replaces the roll for Stabilization with automatic success as usual).

Cool 👍🏻