actually, none of those bands you mentioned are "goth." goth is not a music style, it is an image. GothIC metal is. Tristania, Hagaard, Trail of Tears, Theatre of Tragedy, etc. play music in a gothic style. the vocals have been commonly coined as "beauty and the beast," because of the use of both female soprano/alto and growled male vocals. Therion has none of these charactericstics, save for the use of violin, which is a staple in gothic metal. Therion uses all modern orchestra instruments as well. at least that's how i see it. you are right about it coming down to personal ideas of what a word such as "goth" means.
ummmmm the site went a bit weird for a couple minutes. probably why you ended up with the multiple posts.