NPR and LPR can be catch-alls for all sorts of interesting stuff. I always check both, and always read rate rules closely. If it looks too good to be true, then it is probably too good to be true.

No, hotels don't put Marriott Rewards availability under NPR, so booking a random $0 rate and then handing over a standard rewards cert probably will not work. I assume the only reason the OP is doing this to begin with is because the award wasn't available via standard methods.