Chug, refusing to accept a mission on grounds of aircraft safety, be it lack of self-protection or airworthiness is a very difficult thing to do.
Something similar happened on the Herc Fleet a few years ago and it all got very messy. Careers can be ruined, and lots of questions raised about moral fibre etc.
In reality, if the IPT, (in this case), declares the aircraft airworthy, the AOC is happy to state it is safe enough to fly, then it is very difficult to do otherwise. That sadly, is reality.