"Can a pilot legally refuse to accept an amended ATC clearance if in his judgment as PIC the amended clearance might jeopardize flight safety?" - Bernard F.
“You, not Air Traffic Control, are the pilot in command of your aircraft and are responsible for the safe conduct of your flight! YOU are the final authority as to the operation of that aircraft.
If at any time ATC asks you to do something that you feel jeopardizes your safety, simply reply UNABLE.
Obviously this might create problems for you or ATC, but the key word is SAFETY and that overrides other considerations. If you report UNABLE TO COMPLY with an ATC clearance or request, they should work with you to accomplish a satisfactory outcome.
In all cases, you as PIC have the right to declare an emergency when a distress or urgency condition is encountered (Aeronautical Information Manual Chapter 6). That allows deviation from any rule in Part 91 to the extent required to deal with the emergency.
Let me just add that if you were responsible for creating the situation that led to these issues, there might be an on the ground follow-up by the FAA, but this should never deter you from acting as the PIC of your aircraft!”