I should have added to the previous post a further difficulty with the Rambam based on Kesubos 12: a woman who has had chuppah but no yichud is treated as an almanah (if her husband died and she remarries) and receive only a 100 zuz kesubah. According to the Rambam who holds that nisuin=yichud, the first marriage done without yichud was never completed. Why does this woman not deserve a kesubah of 200 zuz?
The upshot of all these sources is that acc. to the Rambam you have to posit different definitions of nesuin in different contexts. When speaking about rights or entitlements, mesira l’ba’al suffices for a woman to be considered married, both to her benefit, e.g. achilas terumah, as well as to her detriment, e.g. she would henceforth receive a kesubah only of 100 zuz and her property can be inherited by the husband. However, rights and obligations alone do not define marriage – nesu’in is incomplete until the marriage can be consummated, and in that context the Rambam demands yichud hara’uy l’bi’ah otherwise “harei hi k’arusa adayin”, meaning the couple cannot yet live together.
This resolves (I think) the Ran’s question from the din of chupah l’pesulos (Yevamos 57). Chupah in that context must refer simply to the legal status of nesu’in; a husband can never have a legal right to live with pesulos. The chiddush of the Rambam is that even chupas nidah, where there will be a heter to live together, is still invalid as a matir to live together so long as at the time of chupah there was an issur yichud.