I would love the same mod as the stores I look after take preorders up to 2 months in advance.

Also, because of that long time, is there any way to make it impossible to check out with a mixed items cart - so if you have in stock items and preorder ones you would not be allowed to check out?

In addition, I how to restrict the ability to check out with preorders with the same month of delivery? Let's say we take preorders with May and June delivery dates and we don't want to have them mixed together on one order?

Simple request not to mix preorders with in stock items is by default ignored.