I don't like this motive thing. It seems to me there's only two people who could have hated Erin enough to commit such a heinous crime, and only one who did. One of those people was fairly quick to forgive, and seems to have an alibi for that day, so the other one deserves a closer look. This person hated their spouse and the cheatee, couldn't stand to look at them anymore, and wanted to see someone suffer for these misdeeds. Maybe they even found out long before their spouse even knew they did...
It would have been pretty simple, I mean, after a hard days work of offroading and target shooting, "here, let me clean out your car for you"(pocket some casings), a few days later "hey, before we go out I just wanna check my email real quick"('How to dispose a body' click-Search), Maybe a week later, "How 'bout you go ask John 'how to get rid of the body'; it'll really freak him out, ha ha". Smooth things over with Erin, get her cell phone when she's not looking, and send a few texts to an old friend, then delete them. Rent a car for a week or so, making sure it's one that has a 106" wheelbase and close to 60" track, hide it somewhere. {I know what kind of tires he has on his Jeep, I see them every day} Buy some tires, get a trusted friend who doesn't like Chris very much to help swap 'em out. Tell Erin "let's spend a nice day together, away from the third and fourth wheel", meet up with the new car "I've got a surprise" (Red, Jeep wheelbase, and same kind of tires) and when she gets in the car.... instead of taking her out, take-'er-out. Send a last text to Tennessee.....
ADDING: { Chris said something about collecting old tires and metal for scrap, better use this info} throw rebar and tires in after her.
If you're motivated enough this is a very plausible operation.
She sure could have made better decisions..... Also, killing someone is not a good divorce alternative.