I just completed the quest where you summon the unbound dremora and that unlocked a lot of new master spells in Phineas' shop, but nothing actually new. I'm a little disappointed, and I've got some questions.

1) My conjuration isn't exactly 100 yet, it's more like 98, will anything else unlock when I reach 100?

2) is flame thrall any stronger than conjure fire atronach, or does it just last longer?

3) is there anything I can summon more powerful than a dremora lord that I just don't have yet?

Spoilers, but anyway, you can get thralls for each element. The only thing more powerful than a dremora is two dremoras. Or maybe a Markynaz.

yes but is the thrall more powerful than the temporary version of the spell?

Yes, a thrall is more powerful and last forever until you cast a summon for the third time to undo the thrall. Undead Thrall has a lot of potential since you can get hold of powerful NPCs, give them extra 100hp, and they retain all their abilities.