For your first question (2 versus 4 gearboxes), Brandon's reply concerning weight covers a large portion of the correct answer, but there are nuances.
- Another clear advantage of 2 gearboxes & 4 CIMS is that if the front or rear wheels of your robot become under-weighted (and therefore lose traction, such as when climbing a ramp) the drive power becomes available to the other, over-weighted wheels. So you do not lose effective drive power when the weight distribution to your wheels shift.
A second advantage of 2 gearboxes is that you can better afford the weight (and cost) of a shifting gearbox.
Some drivetrains, however, require independent drive to function. Mecanum is a good example of such a drive.
I have nothing new to add to the Mecanum subject.
Regarding pneumatics, it is possible to have a position control on pneumatic devices, although the devices for FIRST are designed for binary (extended / retracted) use. In the chemical process industry (I am a Chemical Engineer), many flow control valves are pneumatic and they control flow by controlling the valve stem position. They work very well & reliably. Such analog pneumatic devices work using a controlled analog pressure signal which varies between 3 and 15 psig. I/P (current to pressure) converters convert higher pressure instrument air to this 3-15 psig signal proportional to a 4-20 mA input analog electrical signal.
In FIRST, however, our pneumatics are designed for on/off service. It is possible to stop at an intermediate point with the standard parts, but it is difficult and complicated. We've done this in test-beds, but have never incorporated this concept into a working robot.