Green Faith Acolyte pretty much gives you nothing in exchange for a weaker companion and worse wild shape. I'd ask to retrain out of that prestige class.
Share spells means any spell with a target of 'you' can effect your companion. You also might check with your GM, but I personally would allow spells with a target of 'your animal companion' to affect your plant companion (although not the general target: 1 animal or things like that). Of course if you don't take GFA you won't have a companion or need to worry about it, and whatever you do, even with Boon Companion your tree friend is going to be too weak to contribute in a few levels if they aren't already.
Druid isn't a particularly strong support caster. Like all 9th level casters, you can do it, but both cleric and wizard have better support options than the druid does. Druids are pretty good at battlefield control however, with quite a bit of terrain options as well as the summon nature's ally line. If that interests you, taking the summoning boosting feats is pretty strong. One big advantage a druid caster has is the ability to wildshape into a small and unassuming creature and avoid notice, treesinger limits that ability greatly since even little plant creature are likely to be weird enough to attract notice, but if you are just casting spells, everything else being equal the smaller you are the better off you will be.
Yeah i did mean by support buffs/crowd control rather than healing. I am considering retraining out of GFA, it's a shame that a lot of prestige classes are bad, as i like the idea of them. Honestly i haven't done a lot of wildshaping yet, i was making a list of plant creatures to try and figure out what ones are good to turn into, my current go-to is the Mi-go, for the flying.