The story about that has been so widely varied that it really sounds like casting about for plausibility given known information, ie lawyer driven.
Now, why would lawyers be all that concerned about when Tracy Martin and girlfriend/fiancee did this or that?
Something else is at play, imho. Sounds like they left her young son with Trayvon Martin and didn’t come back until the following morning, that’s what it sounds like.
This also fits known information and actually does so more plausibly than the “official” Crump lawyer story.