1. Correction: Kreia claims that there's no defense. She isn't omniscient and her word doesn't prohibit the discovery of a defense by an infinitely more powerful and knowledgeable Force user than herself thousands of years into the future.
Regardless, I accept the notion that Nihilus' Drain could affect Sidious as Krayt's did Abeloth; I just don't think it'll end the fight or anything close. No defense does not mean it's an instakill, lmfao. Even without a defense, Sidious can resist it. Defense and resistance are different things. Hence, it could work, but slowly and without any game-changing consequence at first. That leaves Sidious with ample opportunity to strike the Lord of Hunger down.
It wasn't a hundred Jedi, just the majority of "less than a hundred Jedi". Of which we have only three of note. Not impossible to believe, especially since they were taken by surprise.
2. That's an appeal from incredulity. Whatever seems implausible to you is not evidence. The fact that Apocalype was published afters Legacy might be why.