I don't know, I really have no experience with this stuff, but if I were you I think I would be getting a second opinion on the spay surgery. It seems like the most logical thing to me. Many mares are spayed and don't go nuts, so it seems off to me that they say your mare will. Could it be possible that the vet is just against spay surgery?
You could contact mid-Atlantic, as they are kind of in the same "area" as new Bolton, to see what their repro vet has to say. Iirc their vet is supposed to be very good.