Re: DBZ theories and questions three...
1: No. Cell doesn't take on outer appearances of those he absorbs.
2: It's a possibility. But, somethings such as powerful Kai magic may be beyond the dragon's scope.
3: It's a strong chance that each Kai was assimilated into Majin Buu, and essentially they no longer have anything to exist from. They're a part of Majin Buu forever now.
4: Hmm. Well, I suspect that a race as honor dependent as the Saiyan's wouldn't want to use some tool to save their race. Rather live through the disaster as any noble culture would and better themselves because of it. That, and messing up timelines is an iffy subject that once again might be beyond Shenron's scope.