As far as the creaking, there is no damage accumulation. Parts of the plane behave factory fresh until they break. If you're wondering if they accumulate damage if you creak but ease off before they break, I don't believe they do. It creaks first, then if you don't back off, at whatever load it sheds, it sheds.
Relative speed of bullets is modeled. IE- if you HO another aircraft, your bullets hit a bit harder than if you're in a tail chase.
Wiley.