In the game settings, I have "vehicle freelook" deactivated, because I want to use mouse or keyboard to control the vehicles. Therefore I also set the controls for helicopter, plane and submarine in the following way:
control lever up : "UP", "Mouse forward"
control lever down : "DOWN", "Mouse backward"
control lever left : "LEFT", "Mouse left"
control lever right: "RIGHT", "Mouse right"

Now, when parachuting, I cannot toggle between freelook on and off using 2xALT left, and using the mouse does some combination of looking _and_ steering.

Using a helicopter, the mouse and keyboard works as expected.

Btw. in my opinion, the game setting "vehicle freelook" should simply decide what is the default when entering a vehicle, but toggling between freelook on and off should always be possible! Is it currently working as intended (then I will create a feature request) or was it intended the way I think (then I will create a bug report)?