That's right, the diary is important. Surely a diary is the key to getting an inheritance from a rich grandfather. Subsequently, this money will help Fuckface to do the main thing of his whole life - to solve Quinn's problems, and make her life beautiful and carefree. Therefore, the diary is extremely important for this story.
If the MC gets a millionaire inheritance in the future, I don't think his inheritance will matter to the plot. I believe all the inherited money would just go to show that he was accepted by Lynette's father in the end.
Besides, Quinn's or Maya's future cannot depend on the MC's bank balance. In other words, if none of them are the MC's romantic interest, then will they live miserable lives until their last days?
I'm starting to think the MC is indeed "the Choose One". Only the girl who wins the MC's heart will reach eternal happiness.
Even if the option to financially help Maya or Quinn was added to the game, DPC should add a similar option for the other girls, right? But Sage, Isabella, Jill or Josy don't need money, so I don't think the MC's inheritance will have a big impact on the girls' lives.