1. Yes, either leave blank or put N/A - whichever you think works best for that question
2. You likely won't have a home office reference number... they are usually only included on letters from the Home Office sent from inside the UK. If this is your first application from inside the UK, you probably won' t have one.
3. Yes, just put the mobile number for both
4. This question is mainly to find out if he is a UK-born citizen or if he moved to the UK from another country and then naturalised as British. So, I'd put something like: From Birth until X date, then from Y date to Present'.
5. Just leave 3.16 and 3.17 blank as they do not apply to him
6. They are specifically in reference to your current immigration status... they want to know what date you entered on your fiance or spousal visa and started your legal residency in the UK
7.
6.7 - I would put the first date you started legally living together... so if he was legally living in the US with you (i.e. not just visiting), I would put that date
6.9 - I would tick "YES" here because one of the requirements of your current visa is that you have lived together since you arrived... otherwise you would have to explain why you haven't lived together for length of your visa (and if you haven't, you need to provide good reasons why you couldn't live together)
6.10-6.15 - Not relevant to you... as long as you ticked YES to 6.9, you skip straight to 6.16. If you ticked 'No' to 6.9, this is where you would explain why you have not been living with your spouse in the UK for the last 2.5 years.
6.33 - it's up to you how you answer. I think really it's looking for things like legal financial responsibilities that both your names are on and you could both get in trouble for not paying. Things like mortgages, joint loans, joint credit cards etc. However, some people put rent and bills as shared responsibilities. You could always say something like 'I try to contribute to household expenses when I can, though my variable income does not always allow it.'
8. If your husband meets the requirement with only his income, I would NOT include any of your own income on the application form. If you do, you would have to provide all the required financial requirement documents as well... and that would just make your application complicated. Others have said that when they applied in person, the immigration officer only wanted to see one set of financial evidence if one half of the couple met the requirement on their own.
9.
9.12 - Correct - assuming your visa was issued before May 31 2015 and is a 33-month sticker in your passport, you tick NO
9.14 - just fit in what you can of the USCIS address. You can always write it out in full on a separate sheet. You don't need the office title. For example, if you did them at the Miami office, you could just put: USCIS, 8801 NW 7th Ave Miami, FL 33150 - which is only 38 characters with spaces
9.15 - this is the address of the consulate that issued your visa. So it will be the address of UKVI Sheffield.
10. They want any ties you have to the US that might mean you could return to live there - i.e. whether you would have something to go back to if your visa was refused.
So, you could just put: 'friends and family', or you can add more detail if you like.
If you own property in the US, that is something else you could put... just depends on your personal circumstances