But her being #1 in Europe and top 5 in US wasn't in relation to her being "realizable". It was her worldwide popularity. I imagine this is true since they didn't mention Japan (so she probably didn't do that well there), and they had Cloud, who they also could have easily have just said did that well on the ballot and it'd be more believable. I agree that they probably started her in advance, but her being that requested probably made for a good way to sell the idea of her being "the pick". As for what it means for BK, I don't think I'm fit to answer that but I do think if there was ever any Smash game to bring them in more than any other, Ultimate would be the game to do it.
Doesn't it sound a little "fortunate" that the character they chose for likely unrelated reasons (advertisement, easy to obtain, moveset potential) happen to do do that overwhelmingly well? I don't actually believe she placed at the very top considering her game was still new and niche to most Nintendo gamers while only really taking off on the other consoles.
But that made an interesting combination where she had a decent fanbase as a third party but not enough to outshine Inkling, K. Rool, Bandana Dee, Ashley, etc. That's why I'm insisting that they made an educated guess that they could expect her to place decently (Top 20-25 or so), so once they picked out the real top characters for Ultimate (since it was on a superior system with more capabilities), they could be excluded from the tally and Bayonetta could then more realistically jump into #1 for Europe and top 5 for the US. That way, it hides the fact that they were already planning the next Smash game.
Maybe "realizable and negotiable" only meant for Smash 4, and that way you could exclude K. Rool, Ridley, and the like since they weren't going to be in until the next game. Bayo did decently for sure, but we can't just assume that Sakurai was telling only the truth about her performance. I and most others would honestly be shocked if she did that well. Of course, this is all just me speculating and I could have egg on my face if I saw the public ballot results.
Banjo & Kazooie likely aren't directly affected by this, but I do think they were ahead of Bayonetta. I don't know if they'll still fall under a "non-negotiable" blanket since they're a Microsoft property, but I agree that this is their best opportunity to shine.