Honestly, I find that I'm more of a proponent of the "Only through Faith" theory, because that's kind of the one that makes the most sense to me based on the majority of the series and Joss' comments.
I always saw it like this - when you're the Slayer, you sort of have this...I dunno, a "credit" or something that's given to you. When you die, that "credit" is passed on to the next girl, which in turn is passed on to the next and the next. When Buffy died in
Prophecy Girl her "credit" passed to Kendra, which then passed to Faith where it currently resides (although not anymore I suppose, now that the rules have changed).
I suppose the accidental creation of a seperate Slayer line might explain some things about Season 7, but I think it could just as easily be explaned that, honestly, this whole "Two Slayer" thing was an anomoly and nobody knew how it really worked. Buffy could only assume that when she died she would activate another Slayer, and Giles probably didn't know enough to know if she was actually right or not.
However, that doesn't really explain why the Scoobies didn't look for another Slayer once Buffy died in
The Gift, or even considered the option (or why Buffy never went looking for one once she came back, presumably to stop Slayer the Third from pulling a Faith).
I think the inherent problem lies with the fact that Season 7 was riddled with problems that in turn affect and confuse the mythology and things that came before it. If Buffy had never made an offhanded comment about a new Slayer being activated when she died in...oh, say
Potential, this never would've been an issue.
The Watchers' Council, fooled the same as the rest of the world, believed that there was a living Slayer on the hellmouth, never bothered to perform a search for the newly activated Slayer, therefore the girl would not be informed of her Calling.
I always assumed that there was some sort of mystical...er...whatever occured whenever a Slayer died, allowing them to be on the look-out for the new one. I mean, take
Prophecy Girl for example. Buffy was only dead for a few minutes, and even if she saw fit to tell Giles that, I doubt he would have reported that to the Council.
I always assumed that the Council had some kind of mystics attuned to whatever mystical/demonic energy that "called" the Slayer, so they could be notified the moment the current one died and know where the next one would be.
That makes sense to me. I always wondered why the slayer line was unstable in Season 7, because Buffy had already died. Willow creating this alternate slayer line, by accident, makes me understand season 7 better.
I think we should cling to anything that makes Season 7 make more sense to us - because Lord knows it doesn't on it's own.
🙂