1. If we get married in the US, is our marriage license valid in the UK as well?
Yes.
2. Do I have to make 18,600 pound, or is it acceptable for me to be supported by my spouse until I secure employment in the UK?
Only your UK spouse's income is allowed to count for the visa.
You are not allowed to use ANY of your own employment income until you are living and working in the UK and are applying for the next visa in 2.5 years (you can use non-employment income or savings from yourself though).
3. What documents can we use to provide evidence of my spouse's financial standing?
It depends what financial requirement category you are applying under, because the documents required will depend on which income category he meets.
See here for financial requirement information and documents:
http://www.ukba.homeoffice.gov.uk/sitecontent/documents/policyandlaw/IDIs/chp8-annex/section-FM-1.7.pdf?view=Binary4. His employer only provides electronic payslips. Is that a valid evidential document?
The payslips must be original, so if they are printed from the internet they should be stamped/signed by the company as well.
5. The site states that I am required to send 2 recent passport photos. Does that mean that I send my passport photo as well as his? Or, do I sent two copies of my passport photo?
No, two identical copies of your passport photo. The photos have to be of you because they will be used to issue your visa (the visa will have the photo on it).
6. Do I send his passport as well? It was mentioned that I could send a certified copy of his passport. What does that mean and how do we acquire a "certified" copy?
You can send either:
1) His actual passport
2) A certified copy of his passport photo page (I believe a bank can do this, or you can go to a public notary)
3) A photocopy of his passport photo page, along with his original long form (A4) birth certificate.
Some people have been okay with just sending a regular photocopy without certification or the birth certificate, but if you can do either option 2 or 3, that may be preferable.