They were playing as 8’s, not 10’s and Mount was the one more expected to drop back and help Casemiro defensively and when playing out from the back. I think I have issue with people using the term 10 to describe it more than anything.
Let's drop the "no10" term, the bottom line is I never saw him operating deep. The difference in how deep Eriksen operates for us (like a real midfielder) vs Mount is obvious. I think it's harsh to criticise MM for like you do in fragment below:
He didn’t look great at all when being expected to help out with picking the ball up from the defence. The problem with him playing that position is we have nothing defensively.
I don't think he's been ever instructed to do those things. I think the whole ETH idea is that defense + DM will take care of buildup and getting the ball to forward line, and forward line is striker, wingers, Mount and Bruno. That is the bottom line - Mount has been part of the attacking formation, I don't think his responsibility was to pick up the ball from defense or provide the cover for DM. I'd actually say it seems to be Bruno has been the one asked to do those things more than Mount. I believe it's very clear MM has been instructed to roam around the box, but that effectively takes him out of the game because we don't know how to get the ball to the box. Whenever Mount did get involved, I saw some good stuff from him. I would definitely play him a bit deeper to get him on the ball more.
I think the idea that Mount is Eriksen replacement was a figment of our imagination, from day one the midfield was one man midfield and we only dropped this vs Copenhagen, where we switched to classic 4231 that worked well for us. Yesterday after Eriksen left the pitch McTominay moved deeper (again, he definitely wasn't playing CM until that moment) and Mount took a role in between McTominay and Bruno.
Imo ETH got this setup totally wrong, it relies on elite players on the ball in defense and DM, while we should've focused on figuring out how to build up and progress the ball through the thirds using players we have. I think Mount will be useful in that regard, but not if we keep playing him in so high starting position.