I think there were more High Summoners than the series outright states. But this is something that's bothered me for a long time! Because you never hear anything about them, and some of ones we hear about lived a couple of hundred years ago. But that really makes no sense at all, if there have only been four. Maybe the tiny statues in the temples are past ones, I dunno. Haha, also just reread your post and saw you mention the tiny statues.

As for your second question, yes, the game confirms it. I believe it's hinted O'aka and Wantz' sister died during the Final Summoning (although she might have died on the road, like Lulu's first summoner). But the second? time you meet Belgemine, Yuna comments she's so powerful and will surely beat Sin, and Belgemine tells her that she tried but failed. At this point we don't know all that Final Summoning requires, nor does Yuna--she only knows that the Final Aeon will kill her. However, after you beat Belgemine the last time in Remiem Temple, she reveals that she's also an unsent, and having guided you and helped you grow, she feels her failure wasn't a waste. So yes, we do get confirmation that not all summoners who call the Final Aeon necessarily defeat Sin.