First, My using English is not good. Thanks.
In the Vehicle.
指揮官's席へ (move to Commander's Seat) and 砲手's席へ (move to Gunner's Seat)
No need a ''s' and This is need a 'move' word. like a '移る' or '移動' the end of the word is required. I think is the '指揮官席へ移る' or '指揮官席へ移動' and '砲手席へ移る' or '砲手席へ移動' more suitable.
助手席へ (move to Passenger Seat)
The '助手' is meant Assistant in English. I think '後部' or '乗客' is more suitable. Also This is need a 'move' word.
Like a '後部席へ移る' or '乗客席へ移動'.
外側にターン/内側にターン (Turn Ourt / Trun in)
Many Japanese will think of Turn the Car. This will lead to confusion. '顔を出す (Turn Out)' and '中に入る' (Turn In) is more suitable. The '顔を出す' mean 'I put out face to outside from the Car' in English.