Of course that listing agent won't budge, because his/her money isn't on the line.
It might help to get at least 2 more repairs quotes from other GCs, because the lender will probably ignore your brother-in-law's feedback. Yet, if his feedback is corroborated by at least 2 other pros, then the 3 repair estimates should stand, and it will help to strengthen your case.
Do you have BPOs/comps for similar properties that also lack a garage? Did you take any pictures of all of the problem areas?