This is looking like it is pretty close to home... It seems imo someone knew they were going to a horse show (if we assume the guy with the tickets was not involved, he says they never showed) and it just seems to me someone stopped it, knew it or went with... There is also the possibility the one guy killed the other which seems to fit better really and took off... I am also keeping in mind the man they were looking for seen on video with the mother who was a "ranch hand" although they never said he was a ranch hand for any of them as I recall... It sounded as if they questioned and released him...
I'm leaning towards the other missing man did something (just speculation) or this "ranch hand" did or perhaps they even know each other... I'm not sure how the mother plays in but the fact she is missing I don't feel is just an accident or coincidence...
One thing that crosses my mind and keeps doing so was did Grimes bring a load of cash to go to this horse show... We don't know enough whether they were purchasing, selling, purchasing animals or tack...
It was not relatives but I keep thinking of a couple of Wisconsin men, don't remember details, may have been brothers... They had arranged with I believe some stranger selling livestock allegedly a purchase of some cows I think, out of state. I am pretty sure the seller insisted on cash although they may have had a certified check but I think it was cash... A lot of it. They never came back and they traveled out of state to meet the guy. It took some time but it was finally solved. The perp claimed I think they had shown up all went well and they left or something like that... Or maybe he said they didn't show, I can't quite recall... I think they were last seen at a restaurant near their destination too and then leaving to go to the man's property but I can't swear to that...
Pretty sure it was solved and the man robbed them and then killed them.
The reason I touch on this/the point is is I keep thinking why would you kill someone and then take off IF Grimes had a home or assets, etc. but we really don't know any of that... And now the mother too... She could be alive or dead...
So my scenario at the moment/speculation is there was a relationship here and it was falling apart and I am looking at these three as sort of a family... Do we now if Grimes and the younger men were just friends or "partners"? Because it crosses my mind Grimes and the mother could have been bf and gf and that is how the son knew Grime and was friends with him... I don't know enough of the family make up here... In either case, maybe some relationship had been good or maybe one or more had been taking advantage all along financially or swindling the other or something... Or maybe they were just about to lose a relationship, a place to live or who knows... Perhaps the one gets Grimes to agree to a horse show and to buy or bid on horses, etc... Money comes out for purchase to go on this trip...
From there, well, the obvious answer is robbed and killed and then there is money to take off with to start over, hide or hope it is never figured out... The mother may or may not have been part of it...
But I am only speculating and not accusing, it could totally be the other guy they were looking for and the younger man could well be a victim and dead as well and they just have not found him... Or his mom... Adding the mom in is actually what leans me this way... If she went looking for them AFTER she knew they were missing, what reason would any perp have to get rid of her, it wasn't like she found their car or bodies or was in the same location... And the two still missing are mother and son and have a relationship...
I can also make a case on the "ranch hand" guy though too.
In almost any scenario I lean towards, my instinct with the few facts we have is this has a financial motive... Or someone was covering up what they did to another financially...