"Create a newsletter" was a stumbling block to me understanding what's envisioned. I'm trying to think of a better phrase. The problem is, I would understand "create a newsletter" to do something like offering a wizard style interface that creates a page or set of pages, preloaded with suggested formatting based on a template or transclusion...but I think what is intended is more like "adding an existing newsletter to the extension's system." If I wanted to (say) create a newsletter for an existing WikiProject, there's a lot of stuff I'd have to do that isn't included in what this extension is intended to help with.
I agree that this is a general point of confusion.
"Register" would be better than "Create", because that makes clear that the Newsletter itself must exist somewhere.
If we agree on this change, then it should be implemented in every single instanceof "creat*" in the extension.