In regards to your first point, I'd agree if he was failing to produce after having a run in the team. However, because we've only really chucked him on during games where we've been a mess, I'm more lenient. He's not an individualistic player if you know what I mean. He isn't someone who will singlehandedly produce big moments every game. He's a team player who relies on link up play, and, because he's not had the time needed to properly gel with the rest of the first team, it impacts his overall game.
I've said before that I kept up with his Sunderland loan. He was great, but started off rocky as he was still trying to gel with everyone. Once he had a run in the team, and got familiar with his teammates (especially Patrick Roberts), he kicked on. That's what I think he needed here this season. An odd cameo every now and then does him no good.
Ha, I may surprise you but I think I would start McTominay at this point! For two reasons. First being, if he were to start, we'd be keeping Mainoo as our deeper CM (when some of our CBs return), and second being I just think our other options (Casemiro and Eriksen) are horrendous. It's him or Amrabat at this point! (Not really as ten Hag won't drop certain players but I digress!)
As for your last point, I completely agree. Despite being a fan and defending his performances, I don't rate him as highly as you see others do. I don't think he's a world class talent who we would regret letting go of like a Pogba or Rossi (even though we were stacked in attack at the time of Rossi's departure). I do however think he's still a good talent who could not only be a good squad member for seasons to come, but could have contributed something as a starter this season already had the manager trusted him.