I don't spend as much time puzzling over gulls as I feel I aught to. But I've found this to be a useful resource. http://www.birdscanada.org/volunteer/bc ... eAdult.pdf
I think you've reached some good conclusions. Just wondering though, Thayer's are described as having a more rounded head, and this one looks flat to me. Maybe someone wiser in the ways of gulls could comment.