If you take out the center diff, then you will go nowhere, as the diff is required to distribute torque to the front and rear axles. The front diff is also only designed to handle about 30-40% of the torque of the engine, as the remainder of torque is sent to the rear wheels.
There are many vendors that offer off the shelf parts for reasonable costs, I'm not sure why you want to spend more reinventing the wheel and do it yourself.
Try Eric at KitcarChassis in Texas, or Kennedy engineered products.
You will have to either find an adapter or have one machined. I'd start with Kennedy Engineered Products. The 0A2 shares the bolt pattern with the 01E and to some extent the 016. The clutch choice will follow whatever flywheel the adapter plate requires.
It depends on what definition you choose as to what is stronger. Straight cut gears tend to be stronger in static loading, and helical show superior strength with dynamic loading. Ths big difference is if you desire a quite gearset or loud one.
The Front wheel drive 01E is predominantly found with wide ratio gearsets. There are a few weird ones out there that are pretty rare. There is a super close ratio 01E with a .88 6th gear, a 5 speed found in some passats, and a version with a larger ring gear found in some A8's. It is not...
That bolt #11 under the goop, holds on the reverse relay lever #10. Remove the bolt and the relay lever will drop into the gears of the transmission, which will then be ground to dust when the transmission is put to use, essentially destroying everything inside. Of course when customers did this...
It's not a great idea to reverse the rotation on a gearbox with helical gears, as the gear thrust pads are on one end of the gears. Reverse rotation will thrust the gears in the opposite direction where there is inadequate surface area to handle thrust forces.
The 0A2/0A3 are the same design of transmission. 0A2 is 2 wheel drive and 0A3 is all wheel drive. The axle base you speak of regarding engineering of gears is the called the center distance, at least in the USA.
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.