[maths222]

We do option B with mechanically linked motors. Now to answer your questions...
1. B for both (I'm pretty sure for the 1st, I know for the 2nd)
2. Just convenient/saves motor controllers
3. Not fully, as each channel can supply up to 4 amps, and each motor can draw up to about 4 amps at stall torque.
4. The time delay of the I2C commands sending and the motor controller changing behavior is much greater than the time it takes to execute two sequential lines of code, so it won't matter.
5. You should definitely do that if you use two controllers. That has saved us in at least one match. NOTE: plug 1 into nxt port 1 and the other into nxt port 2; that way, if one fails, your drivetrain does not die.