I also feel that it's a relationship that didn't happen but it's a moot point, since there's one lesbian relationship less in the animeverse, q.e.d.

Well, yeah, but the trope's about setting up implications of a relationship, and then not following through. Malicious and intentional lesyay, you might call it. In Canaan, they're both lesbians (or bisexual) and they do indeed start a relationship; they simply break it off quickly because they decide they're incompatible.

So, my question on the definition of the trope: Are Baitand Switch Lesbians present any time two people, regardless of orientation, are built up to have a relationship and then do not, or is it more specifically when two people are presented or implied to be lesbians, but aren't?

Bait And Switch Lesbians is a ploy often used by producers of manga and anime. A lot of people like it when two pretty women act cute toward each other (mostly men, but not exclusively, mind you), so having two potentially lesbian characters will likely garner a lot of attention. But, lo and behold, these two women won't wind up with each other after all, since that would likely endanger the placing in a favorable TV time slot aimed at a younger audience. So, lesbians are being used to lure viewers in (the bait) and excuses are sought not to let them have each other after all (the switch). One or both can be coupled with a male character, be killed off or they simply decide not to be with each other, even though it makes little sense considering their feelings—like in this case. So, yes, the trope is valid.