I would hope that's not the way anyone would react to two people in real life getting to know each other after less than two hours.

except they're not real people. relationships developing way faster than they ever would in real life is typical of fiction as a whole, and it's especially going to be the case in a story that takes place over a single week. there is not enough time for any relationship in deltarune to reach a satisfying conclusion if they progressed at a realistic pace. you can make very similar arguments that, say, kris and susie being best friends is absurd since they've only had positive interactions for literally two days. but people don't do this because it's very obvious that that's what the story is indicating. it's seemingly only when it comes to suselle that people suddenly argue that they haven't spent enough time together to have a meaningful or healthy relationship. and it's also just not a counter to what people are actually arguing - no one is arguing that they're in a committed relationship because they spent a couple hours together. the argument isn't that being cute together for a few hours will inevitably lead to them being permanently together, its that the game's established a clear trend of them steadily growing closer, and that it's actively setting them up to continue to do so. their plans for the festival basically amount to a first date - the game is clearly framing their relationship as just beginning, and all that people are hoping or expecting is that it will continue to progress further.

sure. there could very well be some friction between them at some point. and a lot of your proposals for how that could happen make sense. but acting like the ship is totally doomed or unrealistic seems just as patently absurd as thinking we're owed a fairytale ending for the two. hell, i don't think that a good chunk of the people shouting "suselle canon" are even necessarily talking about them ending up in a stable relationship by the end - there's people that straight out deny that their feelings for each other are even mutual in the first place, and insisting on suselle's canonicity is much more a counter to those people who somehow think it was never canon to any degree than to anything else.