Maybe he lied because he wasn't supposed to be going with her in the first place.
If the training was scheduled to end at 4 pm and they were planning on going together to visit her step-family, why would she call at 4:15 twice to tell him she was leaving town? And police said she didn't leave town until 5 pm, an hour after his training session ended. Why didn't she simply take him with her?
The more I think on this, the more I'm convinced she hadn't planned on having him go along.
He also said she'd contacted an ex in Castle Rock, wanting to come visit while she was there. RS doesn't strike me as the most confident person around. Maybe that is the reason she "relapsed" and he suddenly diagnosed himself as terminal.
My theory:
Things weren't as hunky-dory as he has claimed. She is planning to connect with people from her past, including her ex. Perhaps the ex was her primary reason for visiting Castle Rock... grandpa DID say she'd recently visited down there just a couple of weeks prior to this. She planned to go alone with her son. RS isn't happy, he's jealous, insecure, whatever. He comes up with this cancer "diagnosis" in an attempt to manipulate her into staying with him, to guilt her so to speak. If he's being honest about what she said in the call he received at 6:45, then she clearly believed him to still be at home.
That he was never supposed to be with her that weekend in Castle Rock fits together all those inconsistent statements he's made, and it also explains why he took off on a 2 hour trip not knowing the address of his destination. It would also explain her actions... I never felt she was truly "lost". By the time she started appearing in all these videos, she knew he was most likely in Castle Rock. I don't think she wanted to be there with RS, and started drinking and driving in circles to avoid having to confront him. It also explains why she'd call grandpa rather than step-family. Perhaps she DID decide to sleep it off in the Olympia area in an attempt to avoid that confrontation.