Wow, first of all, congratulations on your engagement
. Second, I really am sorry for all your troubles, man. Thanks for the details. It gives a better understanding of your situation.
In all honesty, the X5 is a great car, and it seems you got the short end of the stick with yours. You should definitely get a new one. It's good that you have extra cars so you don't have to worry when it comes to more important things like work and your every day life. I know what a waste taking your car for service can be. The closest dealer for me to get my car serviced is about 40-50 miles (about an hour drive without traffic). Thankfully my problems were fixed.
If BMW gives you a hard time, which they shouldn't, get a close lawyer friend involved that can advise you in pursuing the most efficient method out of the info you've found and researched. Good luck, buddy!