Thank you for your post.
I wish that I had more knowledge to be able to follow our thoughts a little better. Currently, she has a 403(b) from her current employer, a Rollover IRA (403(b) from her past employer, a Roth IRA, and a Traditional IRA. All these are at TIAA-CREF, Vanguard, and Fidelity (Roth and TIRA). At the moment, she does not have a nondeductible IRA so no Form 8606.
I am not familiar with a back door Roth but I will look into it. I do understand a nondeductible IRA (one of her two options) and she will need to decide if she wants another type of account. But I think that you are saying that she convert it right away (comingle it with her TIRA at Fidelity) and only need to file Form 8606 for this year. Correct?
Since there is still a possibility (I think remote) that she would be eligible for a Roth IRA, can the decision wait until just before she files her 2021 taxes?
Personally, I like the idea of the Roth if she is eligible or the back door Roth (I am assuming that it ends up as a Roth no differently than the Roth that she has with Fidelity). Also, her Roth at Fidelity is very small in comparison to her 403(b), Rollover IRA, and TIRA, all of which will be subject to an RMD.
Have I provided you any more information to opine on?