What is a “studdish face”? IMHO there is no “stallion face”. There can be a stallion-like crest/neck, but not really faces. There are all manners of faces on stallions, from perpetual pony face, to macho face, to feminine face.
I would start with checking his testosterone level, just to get that out of the way, and address it if it’s high enough.
Geldings live with mares all the time. My WB has been in 3 different mixed herds all his life, all very peaceful save for the occasional position challenge (and not always the boys).
One of the worst fights I saw was 2 mares, in an all-mare herd, where one backed up to the other and was double-barreling her for all she was worth - in turnout, not cornered, but they were “anxious” waiting to come in. So it’s not always the boys causing problems either.
Do you have any friends/trainers around who have a super alpha, but super fair horse who might teach this guy a lesson and knock him down a notch or 3? It’s risky for sure, but if it’s possible, that’s a kinder outcome for him than having to be relegated to solitary. Or is there a trainer who can come work with him on your property (since he doesn’t seem to have issues away from home) to really get inside his head and teach him some better behavior? The fact that he’s only “improved” some things by being whacked with a scraper tells me he may have some superiority complex that is still there, he’s just learning which behaviors are not acceptable around you. Maybe your CTJ moments have not really made the impression him that needs to be made. I’m not suggesting that he needs to be beaten within an inch of his life, just maybe some other tactic, or a different approach to the CTJ, if that makes sense.
As long as he’s not really going to kill himself, solitary for now might work. It’s certainly something that has worked for other horses in the past. It doesn’t teach them manners in a herd, but it does help deal with herd-bound issues.