With Denise alibiing for her, that Barb drove home with her, LE would have likely dropped her as a suspect.
Barb could have quickly returned to the VD's though. Barb, had ample opportunity to have unlocked any door and if somehow discovered in the VD house, would have had a ready-made excuse by saying she had left something behind (like her MJ smokes) and come back for it.
This is all speculation, however, I'm beginning to think JJ may have been on to something all along. I do, however, think it's more of a stretch to think that DW would have somehow helped Barb carry it off in any way, though.