With no autopsy, exact time of death might never be known
It said the body was cold, and I think it was implied rigor had started. So he probably died shortly after going to bed.
Pillow could be that he was sitting up in bed with the pillow propped up against the headboard, had the heart attack and laid down in hopes he'd feel better. Pillow fell over him after he died.
I can certainly get tinfoily at time, but in this case I rather doubt it was anything but natural. If Obama (or hangar on) wanted to bump off Scalia now is a bad time to try. Not the worst time to try, but very far from the best. Had this happened when the Dems still had their Senate majority I'd be a LOT more suspicious. Or even just a few months ago when there was still more than a year to the election and it would have been a lot harder for the Reps to argue to wait until the new POTUS was sworn in.