1) Still doesn't really matter, especially if the HD Remixes end up ported to Switch like I suspect will happen.
2) Because it will sell that character as DLC because Sora is probably one of the most wanted, if not the most wanted, 3rd Party character not already in the game. Neku isn't quite to the level that millions of people would spend money on the character alone. 3rd Party characters, aside from maybe Bayonetta, are not included in Super Smash Bros. to drive sales for their series but because of their legacy to gaming and popularity with fans. Bayonetta is an exception only because Nintendo literally had a lot to gain from her inclusion in Smash, Nintendo doesn't have that with TWEWY. Promotion is a bonus since a lot of Smash fans will likely end up looking into the characters they like in Smash's home series, but its not a primary reason for any of the other 3rd Party characters.
3) Nintendo would do so because the DLC character would sell, the characters inclusion could even sell units from Kingdom Hearts fans picking up Smash purely due to Sora's inclusion in the game (this works, after all, many bought Soul Calibur II due to Link's inclusion). Square Enix isn't part of the equation for Sora due to sole Disney ownership (also, they aren't as stingy as people like to say, its mostly due to rights issues for lack of FFVII content), Nintendo is stubborn but not stupid, they wouldn't turn down a deal that would make them a lot of money due to something petty like that, Disney is only strict on their big names, they've licensed Sora to Square-Enix for other projects before, such as Final Fantasy Record Keeper and World of Final Fantasy, they'd definitely work with Nintendo on a series as popular and well-known as Super Smash Bros. The Disney JP CEO also said that they were open to Sora in Smash if the Fans, Nomura, and Sakurai wanted it when asked directly at D20 Expo. If Nintendo was fully in charge, they probably wouldn't have any 3rd Party series character as DLC without some sort of deal made unless they wanted to give fans what they wanted (and thus make more money), which is likely.
I'm a fan of both TWEWY and Kingdom Hearts, but I like Kingdom Hearts more and I definitely like Sora more then Neku, who I'm not really big on (I prefer Shiki). Asking 'isn't Neku good enough' is akin to having Frederik from Donkey Kong Country Tropical Freeze included instead of King K. Rool and asking why he wasn't good enough simply due to being another Donkey Kong big bad. They are similar but not the same.