As a new user, what confuses me is how similar the wysiwyg and the viewer look. IMHO they're redundant, but I get that others may find the viewer essential, of course. But shouldn't it be perhaps more obvious that you are in a view only mode though? Perhaps the viewer should be called "print preview" or "web page render" or have some A4 page preview shape?
Also, how much better would it be if the split view would display both editors (md and rich), instead of md and viewer, so that the user can edit both ways without needing to switch between them?

The three-state switch would not work, as you can change its behaviour to be three or two-way. For instance, the way I have it set up is to toggle between editor and split view only... so in my case is essentially a two-state.