“If they throw Biden in to replace Hitlery, is there any law preventing him from naming Obama as his VP?”
Obama has served 8 years. NO ONE can serve more than 10 years (that would occur if you were VP, and succeeded a dead President on 1/20 of the 3rd year of the term, after 12:00 P.M. Washington time. So you’d have served 1 year and 364 and a fraction days. Then you’d be free to run for 2 more terms of your own. 2-term Presidents can’t do that - they’re DONE.