Could his nibs afford to be seen sheltering a sex trafficker, during the campaign? He could have promised "keep quiet about it now, and I'll take care of you later"

Yes, he probably could have and would have. If she actually had anything significant to trade, her lawyers would not have accepted a promise rather than a plea deal. While a plea deal might have been less than ideal, it would have been plausibly deniable enough that the political damage would have been low (lower than all the photos of him smiling next to Epstein, at least) - so if Maxwell didn't have anything that was worse than that, she doesn't have anything much at all.

And of course, if she had something and Trump broke her promise by not authorizing/forcing a plea bargain while he was a lame duck, she could have gone back after Biden took over and offered to "unbury the bodies" with the prosecutors before her trial in 2021.

Again, it makes absolutely no sense to think she has anything valuable to trade, when her lawyers weren't able to cut a deal with prosecutors - under either Administration. If she had that card to play, she could have/would have played it years ago. She didn't. So the most likely explanation is that she doesn't know where any bodies are buried that the government doesn't already know about.