Encounters: Karrnathi Pretenders

The first combat encounter I'd planned was for the PCs to engage with mercenaries paid off by the Ashbound druids to harry Cyre's coast in order to find a secret House Vadallis installation, and hopefully have the whole thing blamed on Karrnath.  In order to make this work, the Ashbound have created Animated Plant creatures from the Boneash tree of the Eldeen Reaches, a highly flammable wood that resembles human bone to the untrained eye.

When creating monsters, I use the old off-line Monster Builder from WotC, MM3 defense and damage expressions (slightly modified, but not by much), and then I give monsters 4/5 of their expected Hit Points, in order to keep fights moving quickly.

The Cool Thing About This Fight:

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The Boneash Skeleton Warriors are built off of the basic, level 1 Skeleton minions.  I changed their type to Plant and initially gave them a vulnerability to Fire, which I quickly realized was stupid, seeing as any amount of damage on a hit kills a minion.  I wanted to represent that they were REALLY flammable, so I decided to go a slightly different route with their vulnerability: They are so vulnerable to fire that if someone close to them takes fire damage, it ends the "skeleton."
     The other fun thing I did with them, that made the fight memorable, was the Bone Dry trait.  Having them burst into magical flame when they died made for a pretty tense fight in which spreading flames caused some severe battlefield damage-zone nightmares for both monsters and PCs. Rather than roll a D8 for EACH square of fire, I rolled a single D8 each round and expanded all fire squares in the appropriate direction.  I decided, on the fly, that certain forms of damage would prevent them from bursting into flames-Cold, Necrotic, Psychic, Poison, and Thunder.  Untyped, Lightning, Radiant, and Fire damage still caused a blaze.

Other stuff I considered for the fight:

I couldn't find anything in the rules indicating what to do when monsters were mounted, but I really wanted a mounted knight for this combat.

The options I considered:

1. Rebuild them both as a single level 3 elite monster with a shared pool of HP, attacks, and defenses.  This isn't, as far as I can tell, done anywhere else in the rules, but it should, for all intents and purposes, work.  It doesn't handle the PCs doing things like killing just the horse, pulling the Knight from the horse, or doing other cool stunts like that.  This is why, ultimately, I disregarded this idea.
2. Make them follow PC mounted combat rules, and assume the Knight has the Mounted Combat feat.  This uses established rules, which I like, but since it loses the actions of one monster, the XP becomes very inflated relative to the actual challenge.  While it nicely deals with the issues from my initial idea, figuring out the actual challenge to XP ratio proved more work than I wanted to be be bothered with, and the fact remains, that Monsters and PCs are different, and this is a good thing as far as I am concerned.  Thus, this solution was scrapped.
3. Count each monster completely separately.  The horse loses his move action, the Knight gains the horse's speed and the Mount benefit.  Otherwise, they both get their full set of actions, defenses, and hit points.  This isn't explicitly in the rules anywhere, as far as I can tell.  It does have some pretty even trade-offs, and pretty evenly maintains the challenge ratio, and it keeps standard actions for both monsters in tact, making the monsters a solid threat.  This is what I went with, and it worked nicely.

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