I think you actually might over estimate the KH brand name.
I love KH, but it's not as big as people think it is.
I can give a good example right now.
KHUX has had somewhere over 6 million downloads total. That's for global and JP. The game has been out a year on global and 2 years on JP almost,
Idk much about mobile, but 6 million downloads in the span of 2 years seems kinda low.
Meanwhile, Yugioh Duel Links hit 40 million downloads a couple months ago, and hasn't even been out a year. (I had no idea Yugioh was even this popular still)
I've always considered KH a pretty popular game, but with a really loud passion fan base.
I think many people would literally buy some systems just to play the games. (I buy most my handhelds for them.. I would have bought a vita too. lol)
I'd highly recommend watching all the games. Unless they plan on having a 2 hour intro of previous games or something, this game is going to be on par of MGS4 fan service/story. lol
I don't overestimate KH. Hell, i know nothing about KH at all. And i'm not sure i want to, it looks dumb as hell.
I do, however, believe in the strength of disney, and you should too. Disney franchises (specially after acquiring pixar) are insanely popular - they can sell anything on their own. And that includes KH3 on xbox. That toy story trailer alone was enough to convince a purchase for me - focus more on the disney and less on the anime and you can convince way more people. If this game includes a Frozen world as well, it will sell well on anything.