Good point about a opposite situation - Fiat box with VW engine. I missed that part of your diagram.
And I'd forgotten about the certification process where you live. That certainly makes a big difference. Similar to where I used to live in the state of California, where the laws are pretty strict and doing swaps is difficult. I'm sure it would be much easier to do a gearbox swap unnoticed by the inspector than a engine swap.
Good to verify the VW box works directly with the X axles. That makes things MUCH easier (and affordable) to do.
I may still consider swapping my extra VW engine and box into my other X some day. However mine are the earlier VW versions, so not cable shift, and not quite the same level of performance as the 1.8T and newer model box (but still much better than the X's).
The newer cable shift VW gearbox has been swapped into the older non-cable VWs by many people. There are even companies that make a bolt on kit to do it. Look for some of those examples for ideas on how to approach it. I'll see if I have any links to some (not sure).