Did Christina REALLY forget her house key? IF she did was there talk between the two of them on the walk to the garage about this. Did Christina have more than she would normally drink since it has been said she is not a drinker due to her migraines/medication? ("Too much" is a relative term. She may have not been drunk, but if she had, say 4-5 drinks over 5 hours, she might not be drunk but not being one who drinks, she could have been uncomfortable driving. She is also a tiny lady - under 100 pounds.) After discussing this there is a flurry of calls/texts to HF just before they get to the garage. No reply. So she leaves with him in his car. But maybe not to Ft. Worth if HF is not answering. Why go there if he hasn't answered? So he tells her he will take her to her family in Allen. (That would explain why she isn't alarmed when they go in the direction they do.)
Something happens after the toll booth where the car is caught on video. I'm thinking something happened between them (maybe he tried to make an unwanted and agressive advance) and it got ugly. IF he had a bite mark this is where he got it. This is where he got the bruises. And maybe he wrecked during this or later while trying to leave her somewhere.
Just a theory.