A B2-visa is still OK for May. Read thoroughly, including how to expedite an appointment. No agency required, you and your gf DIY by using that wedding thingy. Start doing things now though.
Correct. Unless, he would start looking for means to get a special schedule like now, but really, likely unlikely under the rules and procedures. Too late indeed.
It's stamped, so it's legit? I think it still goes technically right with "must inform the immigration of any change ... " so long as it's timely done.