My thoughts are she stopped some place to get water and was in her way to the house when she got the message to meet him at work. I think he called work, found out they didn't need him and headed that way knowing she was on her way. With both of their cars being distinctive maybe they met on the road near his work and pulled into (a park, parking lot, something near his work) with Crystal thinking he had the kids with him so she could get them she followed him. They got out of the cars, got in an argument and he killed her and dumped her body. This still gives him time to take her car and dump it, run back to his car and go home. If there was a sitter he could have told her he got a call on his way to work not to come in.
Just an idea I've had since the beginning.