PLEASE STOP TAKING THE ENDING SERIOUSLY...
First off, I feel that the show was both real and at times fake (for lack of a better word). It was pretty much the cast's real life's bring documented (much like real world), but at the same time they knew they were on TV and probably got a push here and there from the directors/producers.
Second, the show's ending needed some type of "wow". Kristen getting in the car driving away from Brody wouldn't have done it, that would have been the same as them just ending a normal episode.
Third, the actual ending making it look fake, did two things. First, it kind of pooked fun at the fact that alot of fans said that it was fake and second, it was just a "artistic" way to end it. Think of the final season promos when they were all like puppets on strings, they weren't saying it was fake then, it was just an artisitic way to plug it.
Silly girls...
From a 26 year old male