wrt whether an engineer should say no when told to sign off as safe a system that he/she knows to be unsafe, what is the point of calling yourself an engineer if you could even consider doing such a thing? This isn't about you, it's about your duty to those who depend upon you with their very lives that you do the right thing.
UK engineers in the MOD forgot this when ordered to suborn the UK Military Airworthiness Regulations and sign them off as having been complied with, when they knew that to be false. The result has been a series of airworthiness related fatal air accidents that litter the military aviation forum, a dysfunctional Military Air Safety System, a compromised Military Air Regulator, and the loss of two entire fleets. Over a 100 deaths resulted as well as the loss of Chinook, Sea King, Nimrod, Hercules, Tornado, and other aircraft and systems. We have one member who defied that order, faced persecution, poor health, and loss of a career. He has campaigned for UK Military Airworthiness Reform ever since.
Never mind about Trade Unions or blaming others, at the end of the day you have to face yourself in the mirror every morning, you have to sleep at night, you have to do the right thing whatever the personal cost.
....or don't pretend to yourself that you are an engineer. You are not!