4 things:
Check the pressure control valve in the crank breather system (n249? If so, in line, but bypassed)
Check the MAF wasn't manufactured in 2001 (already tried a different one)
Go to seat and see if there is a software update for your ECU that will cure any intake air temp related issues- does this cost/how much?
20vt engines don't use VNT turbos
Fault code 17705 was created to test every mechanics metal, I've come across it numerous times and always found the cause which seems to be different every time.
I guess seat are likely to charge for that...?