I'm trying to argue (and this is a prescriptive rather than descriptive argument, because obviously people are using "slash" in all SORTS of ways) that slash is a particular kind of erotic fanfiction which definitionally involves a NONCANONICAL relationship.

Except that's not the usage in current practice in the slash community, and I think this is part of what's tripping people up in this discussion. Your definition not only discludes the work that seems to have started this round of the slash debate, but also discludes several works recognized as slash within the slash community. A lot of us are resistant to that, even if we see the arguments against calling that one work slash.