1) Amad is far, far more technically skilled than Cunha though, like by a lot. His skills in tight spaces are on a totally different level and while he's not a match for Cunha in terms of running past people who are running back to their own goal, he's much better at beating a man from a standing start 1v1. Cunha is not that type of player and his close control, acceleration and technique aren't good enough to do it regularly. What Amad lacked was physicality and experience, both things you can get with time but his game fundamentally suits playing in a team that controls the ball and if you put someone who regularly offers him an over/underlap then he's deadly because you can't stand off him and if you get too tight he'll beat you. Against a set defence teams will just sit off Cunha and give him a bit of space because his passing won't hurt you and he'll be limited to taking spot shots or recycling possession.
United are already 6th in the league for possession and part of the reason that's not higher is because our forward players aren't great in tight spaces (the bigger factor is a lack of progression from deep, but that's another issue to solve). Teams already sit back against us and we have a lot of the ball. We need players who are suited to that type of football and Cunha isn't. If all goes to plan we're going to get more possession and control in years to come anyway, not less, which will see Cunha struggle further.
2) That's part of them problem, isn't it? Buying a player who doesn't really excel in any position outside of a 3-4-2-1 for massive money and then having to tell a new manager who doesn't want to play that formation "yeah, make it work" is pretty disastrous squad building. Mbeumo at least a manager can stick out on the wing as a wide forward and get some joy out of it, he's always going to have a role in the team one way or another. Cunha has already shown he can't play out wide and he really, really struggles leading the line. So he's left as what? A second striker with average creativity and poor-ish goal output?