T.R is DEFINITELY gone, that has been confirmed. I have not heard anything about Kate Walsh, I believe her show (Private Practice) is doing just fine and her guest appearance last season was just that --an appearance.
As far as Katherine Heigle, she is def going to be gone, but because she was such a raging biatch on set they are not going to confirm it just yet, they are going to have her 'waiting in the wings' for a while and probably do a couple ghost/in a coma scenes next season but then that's it.