I think overall you have something here that will be a great game if you continue with it, the art style is great and the driving is good, as well as the audio fits nicely into this game.
I think for the constructive points it's pretty much what you have raised, a couple of bugs here and there, stuff like the reversing/quest marker sometimes not appearing. I think also it would be good to have a consequence for hitting into cars, maybe like increasing the heat on the organ you're transplanting.
Overall it's a fun demo that fits the theme!