Yeah Eriksen plays deeper but IMO there is no set system. One of the player drops deeper while other is more attacking, in general both of the mids are attacking. Eriksen offers better passing range from deep, Mount offers good pressing in the attack which helps us to regain possession in opponent third, shame we made 0 use of that in last 3 games.
Yes if we score in that Spurs game its a very different story, which is really not on Mount.
Its still a job for Bruno and Case to adapt to two very different styles of team mate. They are used to Eriksen, and they will have to learn to play with Mount.
I can see why Erik wanted Mount, which I have said in other threads and it is, like you said, because he offers something different.
In games where we need to unlock a low block, I think Eriksen will be the better option, but in every other match we need to get the high pressing to result in goals and in that case Mount will be the best option.
My guess (and hope) is that Højlund will help with that.