First Jeep questions:
1. When firing the machine gun from the jeep, you do not use the minus 2 penalty, correst?
2. The machine gun from the jeep is not a crew weapon right? If not, one passenger can use aimed fire with no penalty?
3. If there is only the driver in the jeep, can he do a move and fire action on his turn such as switch to the machine gun than fire it? or can he only drive the jeep? Or does he have to spend his whole turn changing positions?
They never had a clear section on the rules for using the machine gun froma jeep.
If I remember correctly, the paragraph which gives you the jeep also lists some rules on how to use it and its machine gun. Since I don't have the paragraph booklet at hand, my answers to question 1 and 2 only reflect how I remember the rules, which could very well be false.
2. One soldier can fire the MG without penalty.
3. Changing position costs 1 MP, so you can use the move/fire action to switch to the MG and conduct one snap fire with it.
Driving a vehicle is a full turn action, which can't be combined with anything else.
Now for apertures,
1. Are doors apertures?
2. If they are, when to you use firing into building with no aperture? If they aren't, why don't they use doors and windows for terminology?
2. You use this modifier when firing from inside a building to another hex inside the same building and both hexes have no hexside obstacle (such as an aperture) between each other.
Example: Fire from K13 to L13 on map A (the stone building).
Differences between doors and windows only exist in regard to Line of Sight.