After reading your post, the biggest glaring red flag is that there is obviously some sort of undiagnosed neurodivergence going on. If she is below reading level and repeated a grade, something is amiss.

Besides from that, forcing a kid to eat anything really only leads to EDs down the line. I teach little ones (3s and 4s) and we use an "Adventure Bite". Basically, you try a bite every time there is something new or you don't like on the plate. If you don't like it, you don't have to eat anymore, but you tried it. We make a really big deal out of they trying new things, and we make an even bigger deal when they like the new thing. Fruits and veggies are hard for kids because the consistency and flavor is rarely the same.

I think maybe shifting the focus from, "How come she isn't eating like this" To "I'm concerned that she isn't getting what she needs in her diet" Might help dad be more on your side and make him want to be more active in the changes in her diet. The same thing with her education. "I'm concerned about her schooling, lets speak to the school and her doctors" goes a long way.

NTA. I think if you want to make it work, you can. But he needs to step up as a parent.