The long term plan was to play the 4141, with Mount being the one he wanted playing next to Bruno. He's not going to build a system and make long term plans based on an 18yo kid who had played less than 90 minutes of first team football. It was clear that both Mount and McTominay were playing at a similar field height as Bruno was, and the fact that it was completely different than Scott had ever played before shows it was how ETH was actively setting us up to play. That was his system. It was then up to Mainoo to fit into that system, with ETH developing him to do so over time. Mainoo's natural inclination meant he did it in a different way, but he arguably still played more as an attacker than a midfielder. He then pushed his way into the strongest lineup (partly due to his good performances and partly due to Mount's injuries and poor performances) earlier than expected.
I call it 4141 and 4231 due to that being the best description in my eyes to show the difference between how we played last season and this season. It's not a perfect description as there's so much rotation (for example Dalot sometimes inverting into midfield and sometimes staying out wide to join the attack), and there are some games where it's still been closer to how we played last season. But in general last season we were leaving the DM completely isolated, whereas this season it's a more normal setup where he's not being hung out to dry. Hence why we're not conceding 20+ shots every game. It's not the only reason, but that 2nd midfielder helping out a bit more is part of it.
ETH did have a plan. It was just an absolute shit plan, and some of the players he was planning around (such as Antony and Mount) lost their spot to different players that we already had as they, and the team as a whole, performed better.
The changes this season may have been due to ETH realising what he originally wanted to do wasn't working, or it may have been Wilcox and/or Ashworth telling him to dial it back a bit. But Mount clearly did fit the original plan, which is why I don't understand why people (not just you) say that his signing didn't make sense as we didn't have a role for him. We did, but it didn't work.