I think adam has that the other way around. Leons are 5x100 and Audi's are 5x112.
Although some early Audi's were 5x100
 
Yes if you want them. You can get PCD adapters but they are usually 15mm minimum so your wheels will also be spaced out.
Have a look in the readers rides section. I'm sure many people have done it but most go for replicas.
 
I'm fairly sure early TT wheels were 5x100 and will fit straight on; i know the 6-spoke ones are, and I can't imagine they used different hubs on different specced tt's so they all should be :)
 
TT Wheels are 5x100. Watch your offsets though as they are often different on the rear wheels from what they are on the front. My Audi TT Quattro Sport alloys are a lower offset at the rear so they sit out a touch more. Width is 8J on the front and 8.5J at the rear, offset et30 odd front and et26 rear. I have 10mm spacers on the front to even it up:

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