You're welcome Betsypaige. Had Diana chosen to retain her Royal Protection Officers, yes I do believe that they would have forbidden her to ride in the vehicle that night. It's unfortunate that she did not take up the offer.
Regarding Henri Paul being under the influence of prescription drugs and alcohol, unfortunately that was not immediately apparent to the other occupants.
As for the seatbelts, two separate investigations proved that they were in working order prior to the crash. Unfortunately we'll never know why they were not used by the occupants.