With thanks to everyone for getting me this far, here is my final supporting document checklist for a prospective civil partnership visa. Any comments gratefully received. (I've also added prices, which I thought might be interesting, as they show the effect of the current exchange rate.)
List of documents for Submission to Sheffield:
Applicant Documents
1. Priority processing receipt ($793 from VFS)
2. Return shipping label (outward & return shipping labels for package up to 5lbs was $265 from VFS)
3. Cover letter
4. Printed online application form (cost for application was $2,026)
5. Appendix 2
6. Stamped biometrics confirmation
7. Applicant passports – we think it is safest for her to send both her current valid USA passports. She destroyed her previous passport (which would cover the rest of the 10 years requested), but will explain this in the cover letter and confirm it was neither lost nor stolen
8. One passport photograph
Sponsor's Documents
9. Copy of passport photo page
10. Sponsor letter
Financial Requirement Documents
11. Letter from employer re salary and employment, which also authenticates the payslips and P60
12. 7 months of ORIGINAL payslips, or online payslips that are accompanied by a letter of authenticity from the employer (did 7 months to be on the safe side)
13. Print out of online P60
14. 7 months online bank statements that are stamped/signed by the bank on EVERY page (did 7 months to be on safe side)
Accommodation Evidence
15. Official Copy of Title Register from Land Registry showing I own my house (cost £7 and had to be ordered by post)
Evidence of Plans to Marry/Register a Civil Partnership in the UK
16. Email setting a date for an interview with the Registrar (cost £35 to book the interview)
Relationship evidence
17. photos of us together
18. e-tickets from trips to see each other
19. screenshots of email inbox/call logs/message logs