Hallo Siegfried,
Regarding the first: There should not be a limit turning your head. You can turn your head WITHOUT the machine gun and, you can turn TOGETHER with the machine gun.
I do not know how to do it with the mouse only though.
I use TrackIR and it moves my gunner's head freely, and use the mouse to control the machine gun (not free movement).
Regarding the second: Press Shift-F1 and I think it will be the answer to your question.
PS. I think you can also press Shift-F1 at the gunners position as well, it may help.
Viel Spaß...
