Hi,
1. it is possible to put a mk3 dash in (as is a Corrado, TT, Lupo etc) but far from easy. Involves a fair bit of chopping about but hey it's a challenge.
2. Corrado front and rears go in a mk2 ok but mk3+4 will need the subframes modifying.
3. not sure mate...
4. don't get quite what you mean...
jv