I would stop at the windows and upholster in between them.

I have never seen upholstering around windows and think it might look odd.

I would suggest you upholster between the windows and look at putting in some place button so that you simulate tufting. Also be sure your fabric meets safety requirements for flammability.

You don't upholster the wall. You upholster a separate detachable headboard. Upholstering walls is very 1970 Vegas Elvis.

I also have questions about upholstering a wall. I am using lattice on the sides for blind tacking (no visible staples to cover), but am not sure what to do about the top and bottom edges. My fabric is a large-scale pictorial print with one center seam joining two widths of fabric. How do I keep the fabric straight/level as I go across the wall?