You will need either an unexpired Green Card or an expired conditional Green card along with the 48 month extension letter. You should received the 48 month extension letter right after they received your I-751.
No. USCIS could revoke visas, but courts could review those revocations. Seems courts will now have less authority to review.....just my interpretation.
By law, every person who applies for a B2 visa is already assumed to have intent to stay in the US. Having a US GF is a very strong tie to the US. It is always up to the applicant to overcome that suspicion of immigrant intent. I wish you good luck.
You MUST provide the numbers for all 3 years if the sponsor was required to file taxes those years. Otherwise, you must provide the reason the sponsor was not required to file a tax return.