Not being a doctor or a midwife, I cannot give you advice on what do do. I can say, though, that hands and knees has been shown to be a safe, effective way of resolving shoulder dystocia, although you get no guarantees. On the other hand, cesarean surgery isn't risk free either. If you decide on vaginal birth, discuss your plan to use hands and knees with your care provider. Most doctors have never heard of it. Here is a study if your doctor wants to know more:
Bruner JP, Drummond SB, Meenan AL, et al. All-fours maneuver for reducing shoulder dystocia during labor. J Reprod Med 1998;43(5):439-43.
P.S. I hope you can get that baby turned. I would do it sooner rather than later. And you may also want to look for a chiropractor who knows the Webster Technique. (This is a pelvic adjustment that makes more room for the baby to turn.) Care providers who will do vaginal breech births are as scarce as hens' teeth, and the fact that your babies are so big is not optimal for vaginal breech.
Phew! That's great news!
As for hands-and-knees as a birth position, if your doctor is not familiar with it, her saying that the McRoberts maneuver is better may just be human nature preferring the familiar to something unfamiliar. Were I in your position, I would politely insist that she get a copy of the study and go over it together with you. Start with the old sales technique of making a statement with which the person you want to convince cannot disagree. In this case it is, "I know that we both want the safest birth for this baby. I want to make sure that we have settled on the best option for a birth position given that I have had a problem before."