2. Can Cell use Kaioken?
Presumably Kaio-ken becomes unusable once a Saiyan achieves the Super Saiyan transformation, otherwise I don't see why Goku wouldn't have used it (the episode where he used "Super Kaio-ken" doesn't count, that was filler and therefore non-canon) so, since Cell is essentially like a Super Saiyan, I'm pretty sure he can't use it either. Piccolo on the other hand never really achieved a transformation (Krillin called him a Super Namekian once, but that was just because he was so powerful) so it might be possible he could have used it.
Would that make him the most powerful fighter though? I doubt it. You'd have to do the math and get all the official power levels to find out for sure though I guess.
And if you're comparing him to Vegito, no chance.