- I wouldn't just trace an image, but there's definitely nothing wrong with using one as a reference. Perhaps you could trace it initially and then modify it enough that it looks original.
- Sampled sounds aren't allowed, but if you're stuck for sound effects you can use tools like jsfxr, chiptone, sfxia, etc. And they will sound better than downsampled sounds anyway.
- It doesn't need to be authentic sounding gameboy music. Not everybody has the know-how to do this, and the mods wanted everyone to be able to enjoy the jam. Obviously try to make it as chiptuney as you possibly can, but don't sweat it if it doesn't sound like it's coming straight out of GB hardware.
Haven't used Godot enough to help you there. I know it's definitely possible, so hopefully someone can help you out with that one.