For me the relevance is why would she abandon the van if it was in her name. If she voluntarily walked away that is. Which I don't believe happened.
I guess you've never had an angry girlfriend get out of the (her) car you (the bf) are driving and state she is walking home!
My experience in life and in following stories on websleuths is that we often have this expectation that people always behave clear-headed and logically (and make our judgements based on that)... when in reality people do a lot of illogical things... and I'd say that is especially true for younger "madly in love" people.
The boyfriend's story is going to be the #1 clue.
I think the mother speaking to Gabby on the 24th is also pretty important. It seems to me some of the 'sleuthing' of social media posts made before then, etc.. is probably not going to turn out to be particularly relevant.