The citizenship application has hardly any required documents at all, so you can leave most of the upload categories blank.
I would go with:
Proof of Application- US passport (all pages)
- 2 referee declarations + passport photos with name and DOB on back (not sure if these go here or in the Other section)
Proof of Identity / Travel History- Current ILR BRP
- Sponsor's UK passport
Residence in the UK - not sure if you actually need this, but you can include it
- Letter from my employer to prove residence and also steady income over the last 5 years
Other- Marriage certificate
- Life in the UK test pass reference number (which I think you just enter on the online application now instead of having a physical certificate)
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Proof of living in the UK for for 5 years if applying in your own right or 3 years if applying as the spouse of a British Citizen - what is this proof?? I have a letter from my employer stating I have been employed with them since 1 December 2016. Is that enough?
To be honest, I'm not quite sure what you need, since in the past, you didn't need any proof unless you were self-employed, in which case you provided your HMRC tax returns.
I think just your ILR BRP should be enough normally, but a letter from your employer could be good too, as I think someone said the other day that they were told they needed more than just their BRP.
Proof of freedom from immigration time restrictions - Not sure what this is???
'Freedom from immigration time restrictions' just means that you hold ILR and are therefore eligible to apply for citizenship... so you provide your BRP.
One proof of British citizenship for husband (such as a passport or UK citizenship - Have this
certificate)
You just need his passport.
No birth certificate is required unless he does not have a UK passport.