1: Official Forms (Sponsorship Undertaking form, including the second page for use by the Home Office).
There are no official forms for you to include.
The Sponsorship Undertaking Form does not apply to fiance or spousal visas, so you can take it out. Your fiance should be filling out the online application and Appendix 2, not you.
2. Our Relationship
a. History
Evidence: Paragraph explaining how we met and how our relationship evolved.
b. Choice of UK over US
Evidence: A few bullet points of text.
This sounds like too much. The choice of UK over US should just be one sentence within the paragraph.
For example,
My fiance and I met on X date and (if met online) met in person on Y date. We entered into a relationship on Z date. We keep in contact by (visits and other means of contact). We got engaged on A date and wish to marry in B location on C date. We have decided to settle in the UK because (your reason).
c. Photographic evidence
Evidence: 11 Photographs taken in different locations across the 10 years we have been together.
Take out all except 1 or 2 photos. You only need to show you have met at least once in person.
d. Proof of ongoing contactEvidence:
Screenshot (x1) of MSN Messenger Conversation from 2007.
Screenshots (x4) of email inbox, showing emails from 2006-2011 (approx).
Screenshot (x1) of our Facebook engagement announcement, showing dates etc from 2014.
A selection of physical cards sent between us over the years.
Screenshot (x1) of a Skype conversation from November 2016.
Screenshots (x9) of WhatsApp conversations from various points within 2016 (can't see date on 3 of them).
My mobile phone bill from July 2016, demonstrating me calling my fiancee.
Multiple (x6) landline phone bills from throughout 2016 demonstrating me calling my fiancee.
This is all good, but if you can, don't include any actual conversations in the screenshots - just dates and times is fine.
3. Financial Requirements
Evidence:
Signed letter from employer detailing my employment, including salary.
Previous 8 Payslips.
Bank statements for all accounts across at least the required period (6 months).
Just make sure that you have 6 full months of payslips and bank statements and that every single payslip shows on the bank statements.
I would also include your original job contract and latest P60. Now mandatory, but the guidance says they may ask for them, so it's a good idea to include them anyway.
4. Accommodation
Evidence:
x2 Utility Bills, one from 2016 and one from 2017.
You don't need any bills as they do not prove homeownership (anyone can be on them).