] I think that slip is very telling. They are both very affectionate, but Frederick chooses to stay away, he leaves Eleanor along with her father. He doesn’t even write!

] I might forgive him for flirting with Isabella, I can understand his running away from his father, but I believe he neglects Eleanor. That is unpardonable!

I don't think we have evidence that he neglects Eleanor. Yes, he must be often away with his work. And I doubt that he enjoys being home with his father. But Eleanor likes him (as shown by how eagerly she rushes to welcome him when she thinks he has arrived) and I expect that they do write, although we don't hear of it. We only see a few short months of their life. Frederick stays with them in Bath and is expected to visit at Northanger Abbey. I get the impression that Frederick is usually with them as much as he is able.