Q1: Besides the marriage certificate we submitted: photos for the last 6 years including wedding pictures with both sets of parents and relatives present; joint bank account documents, joint car insurance documents, joint equity line documents/bills, recent overseas flight itinerary with both of our names, Driver licenses with our address, etc.
Q2: If you have an A# it is listed on your H1B card. That's the card that comes as the bottom part of the H1B approval I-797 Notice of Action.
Q3: N/A in our case.
Q4: If you can answer YES to all 3 questions on the first page of the 864EZ then yes, you can file the EZ.
As for the earlier question regarding the USCIS office, based on my research that's kind of an obsolete question, but I did put "Cleveland, OH" there because that's my local office we belong to.
I'm not a lawyer, this is based on my personal experience.