Well, as others have said previously, "here we go again".
I have to admit the issue does interest me, so I'll take another go at it, and hope something new comes to light.
One thing before we begin in earnest though. There's this...
Pagan said:
Can't remember which of the writers it was (probably David Fury, definately wasn't Joss though) that explained about this situation but it wasn't so much a mistake as a simplification for the sake of storytellin
We'll need a link on that. In the past people have posted info they thought they heard on this subject and when it was checked into the quote either didn't exist, or it was made pre-S-6.
The central premise of the "line re-activated in Bufffy", viewpoint is that it happens after S-6. Up until that point, yes the line passes through Faith, and only Faith.
As Primal Slayer pointed out "Joss explained it". He explained it in an interview that happened towards the end of S-5. The link to that is in one of the other threads that have been linked to earlier on this thread. Marti Noxon also explained it previous to that. I think that one's there too.
If someone in the know said anything after S-6 though, that states Faith and only Faith can pass on the slayer line in S-7 we're still waiting to see it.
S-7 does appear to say the rules concerning the activation of the slayer line have changed since Willow's spell in the Bargaining. But we'll get into that once I see if this David Fury quote exists. That could change everything.