it is an ethical impossibility for her to publically endorse him for a second term

I get your point, it is a tough spot. But when you introduce that stance to reality, it quickly goes from “not supporting Biden” to “allowing Trump”. Biden not being the ideal doesn’t mean the alternative is better. Between the two, Trump will cause considerably more harm to her cause than Biden ever would. I am not saying Biden is the solution, just that Trump is worse for her ethical issues. That’s the choice. Sucks but… that’s the choice.