I hope you're right with this, but you've put your finger right here on what's worrying to me. Why Sephiroth and not Aeris? There is just no way to know what "We will never introduce characters that would destroy their meaning or story" and "We will not use characters that are already dead or whose role has ended in their title" means concretely. Especially characters "whose role has ended in their title." That's a baffling criterion to me and sounds tailor-made to allow anything to fly with the right sort of justification.
Like, as an example, I'm struggling to understand how the example Yoshi himself gave, Red XIII from Final Fantasy VII, is different from Lightning. Why would he not include him in FFXIV? Not that I want him in the game--quite the opposite--but I'm wondering why Yoshi has disqualified one and not the other. Who's to say that Red XIII's role "has ended in his title?" Does Yoshi just mean games that aren't finished yet (Like Lightning Returns)? I really don't know. But why does Sephiroth keep rising from the grave when he got killed at the end of Final Fantasy VII? I really don't know. (Unless I'm being cynical and say because fanservice.) So again, my point is this:
It looks like Yoshi has given us a safeguard that he will preserve plot and character integrity; but this is nowhere near an objective criterion at all. It's basically him saying "come on... trust me!"--and so it's worth precisely that much.
It's also been part of my argument that these crossovers do, by their very nature, damage the plot and character of the respective works involved; and that to properly preserve the integrity of the story one must either ignore them or look upon them as deuterocanonical. So Yoshi's response that Lightning is the one and only Lightning and not a copy, and that the writing team worked hard on the quest story--all of that is I suppose a good thing for the team implementing all this, but really misses my fundamental worry entirely.
In short you have a hermeneutic of faith for Yoshi and his team based on his track record, and in truth you're right and it deserves to be noted. He's made FFXIV into a great game, I agree, and while it's not up to me I do think it'll succeed. I'm proud of him.
I, however, have a hermeneutic of suspicion with this whole crossover business, with all of it. Laying all my cards on the table, and with the understanding that I can't prove any of this or don't know for a fact that this is the case: I think that Square-Enix decided, at the upper level somewhere, that this sort of crossover was going to happen as part of a more aggressive marketing campaign of Square-Enix products and titles, and that Yoshi got told that this FFXIV crossover was going to happen. Or maybe he agreed, or came up with the idea himself; I've no idea. But what's clear to me is that Yoshi and his team sat down and said "ok... crossovers are happening. What's the least painful way for this to happen with respect to story? How can we make this happen?" rather than the other way around-- that is, the lore team didn't come up with a fantastic story for a Lightning crossover and then go ask Yoshi "hey look at this great idea we came up with, can we please do this, I bet players will love it."