I think you should take a leaf from your own book.... I hope you remember that that was Ultimecia speaking through a possessed Sorceress Edea. So it was Ultimecia that spoke of persecution throughout generations, no one else.
Of course I remember that. And don't twist your own words. You said, and I quote, that "There is no mention of generations of witches being persecuted". Clearly the quote I provided (spoken by Ultimecia through Edea) proves that sorceresses had been persecuted for generations.

As you said, A Sorceress can not die in peace without passing on her power... If there's no one around what happens until she can find a host? I would hate for the poor thing to suffer but... Whadda ya gonna do? I don't really go with that idea but it's something to consider...
It may be interesting to consider, but it is entirely irrelevant to the discussion at hand. Clearly Rinoa isn't the only person alive in the world of FF8, and so we can assume that she'd find a successor eventually. Even if you assume she somehow doesn't, then the Ultimecia we see should spontanously give up her powers to one of the main party members when you face her in her room (not to mention that she should be twisted with pain).

As for not speaking with Ryu, you can do as you please. I won't reiterate the points Ryu made though, and you can't really make a case without answering them too.