1) It is highly implied that Bellicus and Serena can still use their abilities while Ben is untransformed (so only a stream of DNA on Primus is what's "left" of his body), they just needed a tie-breaker. They can create a space-time continuum with an energy wave from within the pocket dimension so time reversing a universe (or themselves since AX's very first power on-screen was reversing a broken dam, meaning it can be used locally as well) should be piece of cake.
2) Wtf, biological death or like way beyond that? Usually you need to destroy his body to destroy his pocket dimension (somehow that works) to destroy Alien X, not sure how death hax interacts with this.
3) see nr.1
4) His reality warping has 2-B range and he can create portals to and from the Forge of Creation which is outside the multiverse, so really he even outranges 2-B's with his portals.
6) Hmm, does he become a part of his body then? If it's good enought to negate what I said in nr.1 then fair I guess.
7) I mean his energy comes from there, since you see the stream of energy needed to create the universe, comes from Bellicus and Serena. Aggregor specifically targeted a baby Celestialsapien who didn't have personalities yet so he wouldn't have to deal with them after absorbing it's powers (granted Aggregor absorbs/can absorb
Energy,
DNA, Mana (
Life Force and
Magic),
Matter and
Powers and Abilities so maybe Yhwach is exempt from it).