It is indeed very interesting to discuss goalkeeper Onana. He plays for Man United now and he thinks he's great, but he's not. This was seen when he made the unreasonable decision to play 2 matches for United before flying to Afcon. If Ten Hag was not Man United's coach, that person would never be able to play for Man United. He's not good enough. The arrogance of someone who hasn't won anything and can't play football is astonishing. The Cameroon coach is aware that benching Onana is a good choice. I also wonder if after the AFCON cup is over and his return to Man United, Onana will again be Erik Ten's first choice. This is an interesting wait for Man United fans. 
Ten Hag believe in him, that was why he signed him in the first place. If Ten Hag remain the head coach of Manchester United, then Onana will remain his first choice keeper because he signed him from Inter because he knew how good he was while playing under his management at Ajax, therefore he will not bench him unless the management of Manchester United demanded him to do so.Leaving his international duty to stay back and play for Manchester United is something that will keep his relationship with Ten Hag strong, and that is probably the main reason the Cameroonian head coach Rigobert Song benched him to show him that they have other competent players to replace him, which to me is a good decision from the management. And they are very lucky to qualify ahead Guinea with goal difference.
You are emphasizing much on the relationship between Ten hag and Onana which is very annoying. The only relation a coach is suppose to have with his players is the professional football stuff. If a player mess up he has the right to talk to the player to change and if the player insist on not changing he can bench the player or sell him off. No hard feelings. In the case of Onana and Ten Hag, if Ten Hag feels Onana is not performing very well he have another option let him try him and see his performance as well.