While I'll give you many of your points, I don't think Sakurai was only restricted to FF properties in his negotiations--more that he specifically went to them for FF. I personally am not one of the people who thinks he went to them for Geno and got Cloud instead. I think he went there FOR FF, as it was a huge franchise he wanted to try and represent in Smash, and he brought up Geno while there. Best case scenario, he went there for both Geno and Cloud.
But my point is that I don't think other properties are quite as "off the table" as you are making them seem. Yes, there is a pattern there, but that doesn't mean it can't change--2B already proved it can. And if anyone can change it, it's Sakurai. As I've said before, I wouldn't underestimate Smash's power as a bargaining chip--even a company as greedy and possessive as Square has to see it would be stupid not to bend and put some DLC out there from whoever Sakurai asks for. Even if they get an eighth of the sales of Smash for their character, that'll still be a fortune. Cross promotion be damned, that's nothing to turn your nose up to.
The question here, I will say for the hundredth time, is if Nintendo and Sakurai want a character in Smash. That has to come first. Quite personally, I think they can make anything they want to happen, if they're determined enough. I mean, that's why Everyone is Here happened.
It's also worth noting...Geno IS an immensely requested character. I can't say if he's a popular as 2B, but surely he's popular enough to get Square's attention. He will sell, and they have to know that.