I don't think it was possible for Michael to go "legitimate" by society's standards, married or not. I think the difference would have been that if Appolonia were a "typical" Sicilian wife of that period, Michael simply would have had less incentive to pretend that he was going legit. Put another way: when Michael is dancing with Kay at Anthony's party, she confronts him with the "completely legitimate" promise of seven years [sic] earlier, and he replies, "I'm trying." If Appolonia were his wife at that point, the issue never would have come up.