I've got a sentence:
The war seriously affected petrol prices.

My question is why not "petrol price"?
My reasoning is that the petrol price used here is a noun phrase.

My second question is about comparison of similar sentences:

-She has certainly made him angry.
She certainly has made him angry.

- I am really sorry.
I really am sorry.

-Polite people always say 'thank-you'.
I always do say 'thank-you'.

My third question is from a sentence as well:
I don't trust politicians. I never have, and I never will.
Here why is I never "have"? What about I never "do"?

Last question is a US diplomat and an MP.
Why is not an US diplomat? a MP?