It sounds like the majority of that rate is for knowing how to navigate an organization still using cobol. A fortune 50 using such a system as its backbone may be less cost sensitive than a FANG with only a small number of engineers responsible for holding the whole thing up.

I doubt if you're going to get that much if you haven't got a decade plus of experience with COBOL systems.

Helps when you still have your dad's notes when he wrote the system ;)

It depends on whether you are paid regardless of the outcomes beyond your control.

Assume standard contracting in the UK. It's always a daily rate, nothing to do with outcomes.