City and MU are rivals and it can be said that it would be very forbidden if players who were at MU moved to City and vice versa.
The traitor stamp is definitely listed because there have been many cases that have occurred and one of them is Tevez.
Yes that's pretty sure because going to city after playing for years in Manchester united mean the player will receive terrible hate from united fans exactly what happened with tevez and many other players so i think Pogba will change the championship and most likely will choose Juventus or Barcelona