Hello,
Just to add something helpful into the mix following my complicated question, we did find a really helpful resource that may help others here.
If you are over 60 and earn less than £20,000 a year in the UK (as an individual rather than household income), you can access Tax help for Older People. They helped us get Beth registered and on track with her UK tax returns and were excellent. They can't help with details of the UK-US complexities, but were very helpful to us.
Best wishes,
Emily