Chances are those parents weren't beneficiaries of Immediate Relative petitions which don't allow for derivatives.
1. is quicker, the parents should also file for re-entry permits which will allow them to leave the US for up to 2 years without losing their LPR status so they can wait with their son for his process in the home country.
It's not a maybe, it's a no. It clearly states that the part of the process the expedite would help (getting your case assigned to an officer) has already happened. The rest of the process cannot be expedited.
You're right, not helpful at all.
That makes no sense, to be honest. She's not required or eligible to file I-751. It's not a question of choice. The correct procedure is to get the GC corrected via I-90.