Hi.
I was a Marketing Major in my Bachelor's and a Digital Marketing Major in my Master's. Although I have zero experience in the gaming industry, I presume the Marketing Department over at Square Enix Japan* has some involvement in the development of games beyond producing promotional material (screenshots, trailers, ads, scheduling interviews). It's possible that Square has a Focus Group* and the KH dev team show them the gameplay they're developing, to see if it's okay with them, if they like it, and if it has something they don't like, to see how they can workaround that issue. This is why some recent games seem to be easy: because their intended audience is a younger one, and they welcome it by making the game easier. This is the reason why the Sonic Colors Remaster is intentionally easier than the original version: to welcome newer consumers. Gamers need to realize that, as they grow, they're still the target audience, but a company needs to maintain their profitability and they do this by appealing to a larger base of consumers, as in, expanding their user base.
That said, let me address your points one by one:
Surveys, or like I mentioned, focus groups. I know companies like Atlus prepare surveys to see what fans want from them, for example, when Atlus released a survey and one of the questions was if fans wanted to see Persona 5 on the Nintendo Switch. Companies have the option of running public surveys or running or secretly. Take for example Persona 4 Golden for Steam. It's possible Atlus held a secret survey process where they wanted to estimate demand for a Steam version of P4G. And for good measure: the game, on Steam, reached 500,000 players in just a month. And surveys are no stranger to Square Enix: remember that Square has made games, with demos, and those demos are accompanied by surveys to see what issues players found with the game. Hell, they did this with FINAL FANTASY XV: they wanted to know what you wanted out of the dev team: more patches, DLC, etc.
This could be a Digital Marketing exercise. I think it's a possibility, but I don't think it's anything major. They could use it to gauge interest in the story, but I don't don't they monitor social media in order to make better management decisions in the dev and/or writing level.
This is another possibility, one more likely than following community discussions. There have been instances of companies throwing stuff around to see if they "stick". There's also the possibility of companies contacting influencers, giving them NDAs, because they want to test the waters for something, but the selected influencer can't disclose it. Let's make a hypothetical scenario where Sega contacts a gaming influencer and gives him an NDA which purpose is to randomly ask on Twitter if fans want a port of Sonic '06. Although it may seem like a random tweet on the web, it's actually a calculated strategy Sega tried, and whether or not it works, that's for them to decide based on fan reaction.
Notes:
* Remember that a majority of times, the branches of companies outside of the main one work as proxies, in this case, Square Enix of North America is more of a proxy for Japan and doesn't do major stuff
* A focus group is a group of randomly selected people whose purpose is to give their opinions. They usually include the expected audience for a product, in other words: the intended consumers of the product. In this case: a group of people that are intended to be the ones Square is targeting to play KINGDOM HEARTS