Depends on the edition. I always run Outsiders as the "You must kill them on their home plane" (well, once you get past the manes n dretches n lemures) but, as far as I know, they didn't need 101 years to re-form, they were just baned from the prime for 101 years unless summoned by the one who destroyed their prime form.

That just sorta makes sense to me, seeing as they are supposed to be the embodiment of whatever ideal they reflect.

And yes, that happened in the Drizzt novels, the Balor from the Crystal Shard came back using Wulfgar's soul as a bargaining chip (though he tricked Drizzt into believing it was Zak's soul)

Well, I find all rules mainly useful as a guideline more than as holy canon. So go with whatever makes our story compelling - like maybe demons need to be summoned by mortals period.

One DM had that being defeated on the prime material plane gave that mortal more strength against that demon - bonuses to hit it, an ac bonus, and spell resistance against its spells. And when "killed" on the prime plan, it had to rest from the psychic drain.

He also held that demons were vulnerable on their home plane when manifesting on this plane. Which kept them from running too amok here. They would more often send lackeys.