Oh dear, things are obviously not getting better. Still using steel nuts on the 3.5 
We’ve just bought a new RX8 and nuts also are steel so we’re going to have to keep a very close eye on them as well. We’re the same as you as regards the cleaning and looking after the wheels so just take it back and insist on having them changed. Good luck 
FYI as far as I know the nuts can not be made from stainless steel, they would be too soft.
We had our MK3.5 in for it’s first service yesterday, I can’t believe 12 months and 9,500 miles have shot by already!
The dealer changed all the wheel nuts under warranty, great result.
soft wire brush halfords do a set of 3 for around 2 quid dont take off the chrome and bring the rust off nicely not found a way of stopping the rust though but only takes 30 seconds to do a nut and worth the work