Just because the car was abandoned in Chattanooga doesn't mean it was driven there by someone from Chattanooga. Nor that it was taken from Carrollton, for that matter. I'm not thinking carjacker -- or even murder, come to that. I'm still thinking medical incident.
I don't have anything as concrete as a theory, but I'm noticing that from 2am when he was seen in the garage to 9am (10am?) when he was supposed to meet with the family is the same not-quite-5-hour interval that it took to get to Carrollton on the 21st (assuming he wanted to get there a few minutes early). I'm wondering whether there was some other problem/interest/business in that two-hour circle that he thought he could get to and back before the meeting, and he had a heart attack or medication reaction or something while he was there.
The most obvious thing that comes to mind is that he realized he had forgotten something he needed for the meeting and went back to Atlanta or to the farm (do we know where that is?) to pick it up.
The car sounds like it might have been in a barn or carport somewhere before it was "recovered." That's why I was wondering about the details of its condition. I'm thinking somebody might have been hoping to sell it to a "chop shop" or collector -- a 1992 Buick LeSabre might sound like a clunker to you, but it's a car many people love and it's on the edge of being a classic. Not quite old enough yet, but that's the kind of car many collectors like to pick up while they're still cheap. Possibly the collector had it, realized it belonged to a missing person, and decided he didn't want it any more