The MC does know why she left. He knows that she knows about him and their Mom and that he hurt her because of this. Her leaving is because he drove her away by his selfish actions, and he knows this, that's why in this game he wants to apologise to her and make amends with her, and this is something he expresses in the game itself.
No, she didn't. Grace's job offer was simply the most convenient option for Mia, but even if she hadn't had that, she probably would have left anyway and gone literally anywhere because the MC drove Mia away. Mia left because of the MC's actions and he's fully aware that he is responsible for that and not that Grace stole her from him.
Seriously, he doesn't say or think a single thing that shows him to have any resentment towards her because he doesn't. To say that he does is a massive misinterpretation of the game.
The writer who wrote that note at the end of SwM1 is the same writer as SwM2;
Conviction07. I honestly cannot believe that you would say they are wrong, and that you are right.