Could a non-bitter eggplant become bitter after cooking?
Hello Alexander and welcome! All of the information I've seen about eggplants say that it is less bitter with cooking. So, if it is not bitter when raw, it should not get bitter when cooked.

As you note, there are several ways to remove excess moisture and bitterness. Some of those would be salting, peeling, and removing the pith and seeds. However, most eggplants today are bred to not have the bitterness, or at least not as much.

Always be sure to use young, fresh eggplant as the more they mature the more bitterness they will have if they are prone to it.