the issue of disharmony within Man Utd has already been heard quite a bit, before Onana came to Man Utd, Man Utd management reportedly wanted Emiliano Martinez instead of Onana but Ten Hag did not agree with Man Utd management, so actually the recruitment of goalkeepers at Man Utd was influenced due to Ten Hag's decision, in the next transfer season there is a big possibility that Man Utd will hunt for another goalkeeper to accompany Onana, it is also possible that they will continue to target Martinez and 1 or 2 of the existing back-up goalkeepers at Man Utd will be sold to reduce the salary burden.
You're right. It is an old issue that there is bad relationship in Man United. It is not only among the players but it is also related to coach and Man United management as well. If signing Onana is the request from Ten Hag, Man United management must very disappointed with Ten Hag now. They already suggested a better option for the new goalkeeper but Tan Hag refused it. Now, Ten Hag can see that he chooses the wrong option since Onana often played very badly.
Sure, Man United management must try to sign Emiliano Martinez again in the next transfer window. However, I don't think Emiliano Martinez is a very great goalkeeper since his performance looks not really impressive so far. He got many conceded goals both in EPL and Conference League. Aston Villa probably won't let him go because he is their main goalkeeper.
I see in the news that Real Madrid have already agreed on Mbappe's transfer to Real Madrid, but I can't understand whether these are just rumors, or have they already decided everything? Mbappe will receive 35 million euros a year and will be Real Madrid's highest paid player. And it seems that the transition will only be possible in the summer, when his contract with PSG ends.
As far as I know it is not really clear whether they already get the agreement on Mbappe's transfer. So, we can assume it is just a rumor. But if Mbappe has no contract anymore with PSG, it should be easier for Real Madrid to sign him. His salary will be the obstacle for other teams to sign Mbappe. I think Real Madrid is the only team that has the ability to sign him. Mbappe also said that he has a dream to play with Real Madrid in the future.