Last Biden question: doesn’t resigning and elevating Kamala help her?

No, because it projects further weakness (Biden stepping down means admitting he was completely unable to do the job, as opposed to withdrawing from his candidacy, which admits only to an acceptance of low chance of winning / succumbing to pressure)

Additionally, incumbency has been a detriment in elections worldwide (see: European elections)

Kamala probably benefits from not being seen as responsible as Biden. 

I.e. a clean slate.