That sort of triangle is normal when you use CFE to build sunken rooms. There's no real way to fix it that I know of, but there are some wall paintings that will look a little better. I normally use the simplest wall covers for that area, avoiding large patterns and such. You just have to experiment a bit until you find one that's good.
I didn't really understand your question about slants (sorry - I'm not english native
) but if you explain me what you mean I might be able to help you. I'm not the best builder here but I've been using CFE a lot lately so who knows!