1) But that would take a looong time, against the current upcoming Dragon Quest 11.
2) He is really not the most wanted. Each fanbase thinks that but no one has actual data. No one knows for sure who is the most wanted. Besides, that wasn't a criteria to begin with. You're really sugar coating Sora a lot here, that's fine if you like him but it shouldn't stary you from what he really is to Smash. On a level similar to Shantae if we're talking legacy, less than Bomberman if we talk that too. Similarly the rest of what you've mentioned to be honest.
3) Aren't they having a ton of upcoming Final Fantasy titles to the Switch? why not promote that with another FF rep instead? Again, why not a DQ rep too? You like Sora, you want Sora, that's very fine but it currently doesn't make the least of sense regardless of how passionate you are about him.
They don't have to be, but that's what a stubborn does. We'll see. Only a few days (hopefully) to that. Hey, what's stopping Square from having 2 DLC spots. Who knows.
1) No it wouldn't. Square-Enix could literally have been working on it since the Switch's release an announce it for release tomorrow for all we know..even if they started it today, it still wouldn't likely take more then a year.
2) I didn't directly say he was the most wanted, I said he was amongst the most wanted. This isn't a fanbase bias thing, its a looking at who a lot of people are talking about and hoping for. Sora is amongst the biggest ones brought up. Sora is certainly more popular then Shantae and Bomberman though and Legacy, in this case, isn't measured through time but quality, impact, and popularity, hence why Minecraft can have a legacy despite being fairly new all things considered.
3) They could, but Final Fantasy IX, X, X-2, XII, Crystal Chronicles, or Chocobo's Dungeon don't have characters on the level of Cloud's (Squall would be the one to come closest) and I don't think they'd add another Final Fantasy VII character even if Sephiroth is popular. As I've said before though, characters aren't always chosen to market upcoming games, sometimes they chosen to broaden the fanbase of Smash by including popular characters (such as Cloud) to entice people to pick up Super Smash Bros. or at the very least spend the money to buy the DLC. In fact, this is more often the case for Guest Characters then marketing. Nintendo isn't stupid enough to only look at marketing games, they would look at marketing Smash itself to currently non-Smash fans as well, which a character from a series with a fanbase like Sora's would definitely do.
And again, if Sora got in, it would NOT be Square-Enix representation nor would they be involved as anything more then a consultant because Kingdom Hearts is 100% owned by Disney outside of actual Final Fantasy/TWEWY characters and other non-Disney owned IPs like half of the Pixar content and Tarzan. So having Sora along with Neku, a DQ rep, or whatever would not be having two Square-Enix representatives, it would be one Disney and one Square-Enix.