How do you remove a really stuck shower diverter spout?
Quite possibly. I have cranked on spouts like that and had them release, and had them fold up and crumble, since it was old pot metal. A grinder would be a good way to go about it. The connection, if it is as you say, will be in about 3" in. The rough in for a connection like that is usually 4" to 4 1/8" to the end of the male adapter, so a cut about 3" should get you to the  adapter.