The quattro drivetrain mounts the engine significantly forward of the front axle, which would make engine placement in another chassis very difficult, not to mention the handling drawbacks of having the engine overhang the front axle by more than 2 feet.
For the same tooth profile helical gears will be stronger, however straight cut gears allow the tooth root to be much thicker which increases strength. Straight cut gears are also more efficient as there is no thrust force produced like in Helical gears.
The 5 speed 01E is much more difficult to locate, and 01X's are all 6 speeds. No the 016 is not an earlier version of the 01E.
It' not been done to my knowledge, but that doesn't mean it can't be done.
You can't spin an automatic backwards and have it work. The pump rotation is directional. If you happen to overcome that hurdle the thrust surfaces will be on the non thrusting side of the gear stacks. One way clutches only work in one direction... the list is endless.