Alright alright... I don't have the time to read mega posts like that back and forth, most of it recap. Let's go with what I know... seems nicer that way.
Kreia IS known to the Council. She is referenced to during the Exile's trial. Pay attention.
If Kreia has no idea about Ataru, how can she tell you about it directly? She may be no real practitioner, but she is bound to know it when she sees it, and perhaps even fend it off. She is a bloody Jedi master, you know.
Again, Kreia's Force mastery draws on both sides and is immense. If she can swat Vrook around like a ragdoll (Vrook who was the second in command of the council of the KOTOR era) she can possibly... POSSIBLY very well overwhelm Yoda. After all, Yoda does not draw on the dark side and must find alternatives to counter Sith attacks. IF it came down to saber combat, yes Yoda would likely win. But not easily. Not at all.
About the Exile: he must be reasonably good. I would assume he could (If he actually became Sith) challenge and defeat Malak after a hard fight. Can he take Mace? Probably. Mace might be able to beat him, but I wouldn't bet money either way. I'm gonna leave this undecided until KOTOR III, or else I'd be wasting my time.
About Sion: We never find out of Sion can be bisected or otherwise cut to ribbons. Assuming he can be, but the hatred and control of the dark side keeps him together (Which I believe it does) his very being might unnerve Obi-Wan after the Jedi master finds out Sion isn't gonna die easily. And Sion IS the leader of Revan's dark jedi assassin group. Hence why he is always ordering them around.