I think you may find it more useful meeting with the manager or owner than waging such a 'colorful' display. I think a gentle reminder that they would not want to loose your business may shove them in the right direction. Also, I think they would be much more inclined to trade you another car/X5 rather than buy back the X5. If you tell them exactly where you stand, I really don't see it to be in their best interest to pitch a fit; and chances are these are not unsophisticated business people; they are going to recognize the implications. BMW (and most luxury brands) sell primarily on reputation.
Also, if the BMWNA rep fixes the problem, tell them great. But see if they will sign an agreement which states, if there are any further problems they will buy back the car (or do something which is amenable to you). I find that 'reasonable' people stay reasonable when s/he is given an option. But when forced with an ultimatum or something sounding like a threat, s/he may loose the 'reasonableness' of his/her character and you may end up in a game of chicken.
I've lost my head and not kept this in mind on several occasions, and upon retrospect, I know I could have gotten my way if I was more willing to give a choice rather than insist.
You seem like a reasonable guy, so I'm sure all these words are irrelevant, but help is usually a good thing.
Good luck Adam!
Sean