Maybe Old Ring Hero Radagast's tree (not with summons) with anti poison effect can work. It absorbes nearby poisoned damages and hides units , not necessary to heal units. Also suits for Treebeard's protector role of Fangorn Forest and supports wood elves.
It's not bad idea and nice recyclation ... am not sure about effects right now (central spell and mist hide units, heal can be casted either way by spellbook spell), but trees in the Lorien are in general very tall, I can imagine that Galadriel gives him some modified seed with special tree.
Yes, resistance to poison for nearby units doesn't sound bad.
Problem is that we don't know how the heart three looks like ...
The Central Spell: Elven Wood
Creates a Heart Tree around which a forest grows that hides allies within.
After a while, it also lets them use Ambush of the Wood-Elves even when detected and reduces its cooldown.
That's why I'm suggesting only regular trees. Something simple.
the gem for Thranduil is more suitable. I'd consider ot make him resistant to elemental damage only, it fits even more if we are going to focus him as a monsters slayer too.
No, no, only allied units can receive bonus against those kinds of units. Monster slayer aspect was before unit supporter concept came out - when he was pure mass slayer only.
Level 5: Sindarin Armour - Thranduil puts on his battle armour, which gives him +25% to defense. His silver armour inspires nearby units and grants them +35% defense against monsters, heroic, elite and siege units. (Passive ability)
On 15 March, T.A. 3019, Sauron's army from Dol Guldur attacked Mirkwood. There was a long battle under the trees and the woods were set on fire, but in the end Thranduil was victorious.
In fact I also thought in similar way - about poison solely, especially when "Greenwood is sick", means poisoned, so enchanted gem with resistance to poison was obvious and fitting.
But we don't have proper ingame effect. And if Walkure wants to keep magic bubble, then magic resistance suits only.
Of course - we can search for antipoison ingame FX and move magic bubble to CaH, which was my initial plan.