Can you do something useful with nature trolls?
Basically no, tbh. The nature summon one is 1D1N 100% EDN with nothing going for it except being a troll, whereas the kings are both 3 in their respective paths and gem generators. Their coterie also significantly better than the forest troll one, too -- the forest trolls are basically the worst trolls. There's a reason the forest troll tribe is 37N compared to the courts' 55X, heh.
You could do a little bit with it, maybe -- about as much as you could do with any 2D or 2N caster, if you get lucky and get those instead of ending up with a more or less junk (barring trinket casting and site searching, anyway) 1E1D1N -- but you can't squeeze them for nearly as much as you can the kings.
E: Well, thinking on that a little, a nature nation
could use them to break into death, if they've absolutely got no better access. And a bucketload of conjuration research; the easiest 3D summon that could top a 2D one seems to be mound fiends at conj 7, heh. Maybe a lamia queen, which a 2N shaman in a nature nation might be able to boost up to...
Course, if you've got enough death gems laying around that burning that much N to be able to start spending them is a good idea, you've
probably already got death access. So... yeah. Forest troll tribe isn't the best of unit summoning spells, heh.