I don't know if I buy the mistaken identity /hitman theory totally. Sure, it is possible but would a hit man at the very minimum know his intended victims home and work address? The first MM was murdered between home and work and her car was three miles from her home. Certainly, someone could probably hire some tweeker for a couple hundred bucks to kill someone, but there seems to be some level of planning that went into the first killing at least. One thought that struck me was what if the first murder was done to cover the second? Not a mistake, but someone poor attempt at trying to make it look like , perhaps, a serial killer to throw investigators off the track. Maybe the coworker sought out another woman of the same name ( Mary Morris would not be a hard name to find in a town as large as Houston)? Just a thought that came to mind, probably far fetched though.
One final thought. The second Mary recognized someone in the drug store that scared her enough to make a 911 call. Would she fear her husband that much, on sight, even though she had asked him for the gun? Would the husband hire a hitman that his wife A.) knew and B.) knew was a hitman that was after her? Seems like the co-worker is the logical suspect for the guy she spotted in the drug store. While the husbands actions may seem strange to some, I do think many people would do the same thing.