I don't think De Gea will return to the United as he's comfortably playing for Fiorentina. However, who knows he can change his mind. It may happen as long as United offer a little increased salary for him.
As for that observation, it makes sense considering so many goal keepers, which are 27 - 35 years old range still perform very well. Even some are just getting even better.
Lastly about Onana, they can't only concentrate to him. He obviously lacks of competition. I'd rather try to bring a quality goalie (not De Gea) to add more competition. The bigger pressure, the better for the player.
I think he will.
It will be an epic comeback if De Gea return to Manchester United, especially if he can prove if Manchester United was wrong by making many saves. A goalkeeper can still able to play in top league up to 40 years old e.g. Buffon, Neuer, and Casillas.
Onana will be kicked out after De Gea signed, because Manchester United can just looking for potential goalkeeper rather than train Onana.