Can I prepare sashimi and leave it in the fridge overnight?

Answer 1

pre-cut the fish?

I suggest that you don't. Try to serve it as fresh as possible.
If you do have to pre-cut the fish, I recommend wrapping tightly with plastic wrap. In fact, before you refrigerate, press the plastic wrap onto the fish so that their are no air pockets.

Answer 2

Only if set upon a bowl of ice, covered in plastic wrap, then covered again. Eat within 24 hours of purchasing fish, and make sure it's sushi-grade fish.