wait so the lack of saving was because of refresh rate? that's crazy.
How do you change refresh rate?
the game doesn't have a save feature since its so short (roughly 10-15 minutes), go into your computer's display settings -> advanced display settings -> choose refresh rate to 60hz. This will stop the slimes for bugging you out, casting spells no longer will fail and you can move much much faster (if you don't change the refresh rate getting the item from the bunny race is impossible as even with all the mushroom speed upgrades you won't be able to win).
I changed my refresh rate, and it worked well on making movement a lot more fluid, and it fixed the spells not firing bug. But, I still get hit by the vanishing out of existence when hit by a slime while charging a spell bug. Maybe you could try adding an "unstuck" feature in the menu that just kills you and sends you back to your last used checkpoint.