Those Rick and Morty nods are really nods to the Rick and Morty nods in 'Rhetoricstu does it was all me' rather than the show itself! Not that it makes much difference.
I replayed The Dying Core just to make sure, but it doesn't do rising water in a similar way. The section you're referring to is a remake of one from Cosmic Meltdown (http://nifflas.lpchip.nl/index.php?topic=5855.0) that uses mostly the same art assets, so maybe that's the level you had in mind.
As for the violin cover: it's from Steppes III (http://nurykabe.com/dump/?dir=.%2Fsound%2Falbums%2FSteppes%2FSteppes+III) by Kanar, a band comprised of Knytt Stories composer
Nurykabe and his pal Léo Mathieu (if you don't like FLAC you can get the whole album in MP3 format here (http://www.mediafire.com/?3n3265l0daim3x0)). It forms the basis of one of the radio stations in Saira (http://nifflas.ni2.se/?page=Saira), and I also used a different track from the album previously in the level Rainy Day Adventure (http://nifflas.lpchip.nl/index.php?topic=5217.0).
There's other music by Nurykabe in Juni's Nightmare that you might like to know more about. The first section uses a couple of tracks that were made for Knytt Stories but weren't ultimately included with it. You can find all of those here (http://nurykabe.com/dump/?dir=.%2Fsound%2Fsoundtracks%2Fknytt+stories); tracks 09-13. They're released under an extremely generous creative commons license, so they're a really painless way to give your level a classic Knytt Stories feel without relying just on the default music. If there's any interest, I'll convert them all to tagged OGG files myself and post a convenient download link up in the custom content subforum.
I'm also looking forward to Healy's level! Apparently it could be a while, but good things often take time.