Desire has harmonica in it, doesn't it? They just played that live at iHeartRadio--did Bono play the harmonica then? If he did, I'd say he's good to go for TJT tour.
Regarding nerve damage and guitar playing, if you can't control movement of your fretting fingers, it's almost impossible to get them in the right position to play chords (even basic chords). It sounds like when his shoulder got busted up, it messed with those nerves. I play bass/electric guitar and injured my left pinkie over a decade ago playing football...I still can't use it effectively myself, despite years of trying. I could only imagine how tough it would be if all fingers were as difficult to maneuver as that pinkie, which I assume is what nerve damage is like.
That said, physical therapists can work miracles today. Even people who were told they'd never be able to walk again have done enough PT to get to a point where they can walk. I did a 8 week stint of PT last summer and saw many people come in with BAD shoulder injuries, only to regain most of their movement within a couple months. So I don't doubt that Bono's nerve damage could be reversed somewhat with the right PT team. I think if he really wants to play again, he'll find a way. If not, no shame in hanging it up, especially if it's too painful for him.