Word on the warning for slash (although, I've warned for het on fic before, but really it should be obvious from the pairings listed).

Re pre-slash - in BTVS fandom (Spike/Xander), pre-slash seems to be treated as a category rather than as a warning, where the stories fall into a gray area of not being gen-fic because there's some movement towards possibly establishing a relationship and a developing relationship fic. There's no satisfying conclusion at the end where they are now together, but they're no longer platonic friends. But, you're right, there's got to be a better way to describe that state.