Contridictions of fixed points in Dr Who s6

The Doctor’s fixed-point death in Utah contradicts every rule about fixed points. Fixed points cannot be changed. Yet River's refusal to kill him collapses the whole of reality, but it somehow keeps running in some way? And what about 11's survival? I adore 11. Glad he survived, but it's impossible to have. The fixed point called for HIS death, not a substitute. He used the Teselector to do it, but it shouldn’t have bypassed the fixed point. He needed to die there and then. Later episodes treat fixed points differently again. How does that work out?