I think a big thing IF he were to come back here, would be how he handles crowd abuse. And probable opponent abuse. And online abuse. Because it will inevitably follow him around wherever he goes, but moreso in the prem than elsewhere. So that's why I could see him move abroad for a fresh start. Even years from now in the Prem it would be repeated, I don't see this one dying down really.
If he is to come back at United, my guess is there would have to be a lot of publicity work on the part of (importantly) both Mason and his girl, while also committing to support all *those* causes, classes, donations, whatever. That's the only way the club would be able to actually reintegrate him back in the squad from a social point of view with the rest of the fans and wider world football society. I think there's a chance that if that is to happen then he could be somewhere on the "normal crowd abuse" spectrum, and he could focus on his game again.
First club has to decide of course what to do, and then he'll have to show in training what he's capable of on the pitch. 1.5 years is a long ass time. Loan makes sense from a club perspective in pretty much every single way, bide their time to decide what to do, get Greenwood regular gametime to get fitness, development for a future role or gain value for a future sale, let the situation simmer down... No brainer.