I was not in the Air Force, but I was in the Navy and things are a little different over there. I can only offer my perspective based off what I have been able to research on my own because I also plan to return to active duty as a nurse. From what I have researched, if you are not chosen for the field you wish to go into, you will either be placed on a waiting list or will have to reapply. If you are placed on the waiting list, you will be given a spot in the event that someone cannot fulfill the obligation. This is contingent on how long the list is and who else is in front of you on the list.
Also, you mentioned that you would like to do the mil-to-mil thing with your fiance' once the two of you are married. I can tell you that from a military standpoint, you will not always have that option because it is based on needs of the respective service. They will do what they can to place you within the same vicinity, but keep in mind that operational commitments on his or your parts may not always allow that to happen. I hope this helps.