Risk v reward is a very personal thing, and I too would have aborted the flight knowing that if bad stuff happened I would not be as protected as I could have been.
Imagine someone having to explain to your child that "Daddy didn't come home because his head got smashed in during a crash, and that had he been wearing the full belt he would have survived."
That's my perspective, and yes, others will differ.
As for getting cash back, thats a tricky one as I suspect you had to pay something to secure the booking. The challenge is a purely legal one I suspect, and centres around what is classed as U/S. If the belt is not part of the required kit then I suspect you're not gonna win...