This is all very illuminating in some ways, and these are questions I have wondered about in various (less articulate) ways for many years. However, nobody seems to be citing the enormous Xena/Gabrielle fandom, an interesting phenomenon in its own right which was a bit of an offshoot of mediafandom, but hardly unfindable. (I am reminded, though, that at least one fan I knew at the time automatically considered Xena slsash to consist of Ares/Joxer, until the obvious X/G examples were pointed out to her.)

More generally, f/f as I've seen it has generally been fostered by fans seeing two credible female characters together, and is still minor in comparison to m/m overall. Due to Progress (TM), this has happened mostly in the 1990s and later.