As for the Beckham comparisons and "mental strength" - no, that doesn't work.
Beckham was never hated by a single United fan (with the possible exception of extreme weirdos). If anything, the incident made him even more loved by United fans in a "us against the world" way.
(And yes, I know that some weirdos want us to take a Suarez t-shirt style stance here, but it hasn't caught on - and it never will, bringing him back will be controversial, not only in general, but also within the fan base.)
ETA To be clear, I don't think there would be a huge, lasting reaction * if United decided to bring him back. I'm just saying that the Beckham comparison doesn't work (because the basic premise is not the same).
* Greenwood is a media nobody compared to Beckham, that's another difference with regard to the basic premise (but, I suppose, something that works in his favour if we're talking about how "mentally strong" he has to be in the event of him coming back).