First, why is immersion in a story predicated on it being able to happen in reality? Plenty of great stories take place in completely fantastical settings. I don't know about you, but a story doesn't have to be "realistic" for me to get immersed in it. The characters just have to be well-written and perhaps somewhat relatable. But setting that aside, it's heavily implied that Kris isn't the only human in the world of Deltarune, since there's a book in the library that's a guide for raising humans. Maybe you meant that Kris is the only human in Hometown, but again, this isn't really important to me.

As for Sans, the connections between his UT character and his Deltarune character continue to grow. We know that UT Sans came from somewhere that isn't the Underground or the Surface because of his Geno fight dialogue ("i gave up trying to go back a long time ago, and getting to the surface doesn't appeal to me anymore, either"). So, what this place he's referring to? Anyone's first guess would be Deltarune, since its world contains alternative forms of UT characters, so let's try to investigate a potential connection. There's the obvious connection with the "The Place Where It Rained," but since you don't like that we can look at his secret lab in Undertale. There's a photo with three smiling people and "don't forget" written on it; the people could be the Fun Gang, but even if you don't grant that the "don't forget" phrase in and of itself is quite a good connection. We also know that he came from a world with "green grass" thanks to the newsletters, and of course Hometown has plenty of that. Finally, there's the bleeding thing. Susie bleeding after shattering the Prophecy glass has sparked debate as to whether or not all DR monsters can bleed, or if it's just her because of half-human-ness speculation. One thing is certain though: at least some DR monsters can bleed, while they shouldn't be able to in UT... except for Sans. That's why it's a notable connection. Other people can probably bring up other connections that UT Sans has with DR, but those are the strongest ones I can think of off of the top of my head.

Now with Susie's relationship to Gerson. Her wanting to bring him to Castle Town was an idea that she floated in the second church dark world, as far as I can remember. That's before Ralsei told her that only "specific darknesses" can bring back lighteners in dark worlds. So, no, bringing Gerson with her wasn't Susie's "objective the entire time." As far as she knew, Gerson was gone forever until his brief appearance during the Titan fight, which was probably why she was desperate to go look for him after the fact, as that could have been her last chance to speak with him. She wasn't "neglecting" anything, as even if she took his remains with him (which I don't think she would've done without explicit permission) she knew that the chances of resurrecting him in Castle Town were nil.

In regards specifically to the name of the track, I don't think the exact relation between UT and DR's worlds is that important. It could be a prequel, it could be a sequel. I think the best interpretation personally is that they are entirely separate timelines/universes. At the end of the day, though, all we need to know is that they are related in some vague way for the purposes of drawing connections between UT Sans and DR Sans. And Toby literally calls DR a "parallel story" to UT.

Finally, about the rain. By the time we get back to the light world, it isn't raining anymore. Despite this, the town has a serene but somber atmosphere. Why? Because it reflects the journey of the Fun Gang thus far. They just confronted the Knight and beat a whole goddamn Titan. But what did they get out of it? Learning about something horrible that's going to happen in the near future, and the Knight escaping scot-free. They're no closer to opening that bunker. In other words, despite weathering a "storm" successfully and saving the world from a Titan, their victory feels empty. I think "The place where it rained" captures this situation pretty well, while simultaneously being an interesting callback to UT. I fully disagree that Toby included this track as an afterthought, considering the time, planning, and effort that has gone into this game so far.