Man, I feel for you.
I don't know what it is with women, but when it comes to splitting up, they often just get so freakin' RANDOM. I can't tell you how many times during or after a breakup, when the woman involved has done something so out of left field that I find myself saying, "Where the f--- did THAT come from?"
Doesn't matter if it screws up YOUR life. They'll just do it anyway.
But where your kid is involved, I admire your attitude. If she continues down this "do whatever the hell she wants" road, it's gonna be tough, and I have no easy words of wisdom.
When the crap comes down, I usually just grit my teeth and try to get through the hard times with as much calm as I can muster.
Addendum: some of the other posters are right. A "military brat" lifestyle is NOT stable, so you might have a case for majority custody. Definitely look into it!