There is no single answer to documentation needed for an AOS interview. The more the better. Anything received from a governmental agency is good. That would include the Social security administration, DMV and others.
I would NEVER go near CIS on a spouse AOS case without a good lawyer. They are not out to help you, and there are many, many traps you can fall into.
There are many serious implications dealing w/ the abandonment of a gc and the ability to re-enter later. First of all, for a new gc you'd have to start a new immigration case. You may or may not qualify in the future. I would have a competent professional look at your case now for an assesment and advice about future applications.
Second, if you choose to come back as an H-1B, serious cap issues may occur. That is, all of the visa numbers for H-1B for that year could be used up and you...
Retaining your German citizenship is a function of German, not American law. When taking your naturalization oath here, you are "renouncing prior nationalities" The effect of that renuciation in your German citizenship is decided by the law of Germany. Check w/ the German consul.
By "sponsor", I assume you mean filing an I-864 affidavit of support. He should file all of his back taxes as soon as possible. I doubt the USCIS will accept a filing and give a benefit without the co sponsor's papers being in order. SEE AN EXPERIENCED IMMIGRATION LAWYER.
I agree with the Attorneys who said to get married before the next hearing date and file an I-130 petition.. The Hartford Immigration Court will not necessarily give you enough time to get the I-130 petition approved and for you to apply for AOS. They are very strict that way. I strongly urge you to consult a lawyer experienced in the Hartford Immigration Court. There are quite a few.
Right now, they are issuing visas to people who had petitions file for them on or before 01/15/2000. You should check http://www.travel.state.gov/visa/bulletin/bulletin_1360.html go back a year or 2 and try to interpolate how much time is left. It's not an accurate process, but that's the best you can do.
You can't "age out" as an F4.