I believe each person's IRA is treated separately, so the 2 of you shouldn't have to worry about the other person's pro-rata rules for this backdoor roth conversion, you just have to worry about your own issue.
IN the case of your 401k not allowing a rollover from your SEP-IRA that is a conundrum.
Solution could be to right away start your own self-employed business, and then start a Individual or Solo 401k plan with either Fidelity or Vanguard. First, Make sure that either Fidelity or Vanguard's Individual 401k plan allows a rollover from a SEP IRA. If so then proceed to generate some self employed income, whether you become a dog walker, baby sitter, freelancer, or any other job. Then set up the Solo 401k Plan. Roll your SEP-IRA into it. And voila, no more IRA's to have to worry about regarding your ability to avoid the pro-rata rule for the back door Roth rollover.