1) I have mine lowered quite a bit and also have skid plates. I can hear it scraping all the time, but I think it's the plastic lip that's scraping. The skid plates seem to never have scraped, which is shocking considering how low my car is.

2) My dealer told me I could add the warranty at any time before the factory warranty expires. They told me not to even worry about it when I bought the car, because if I sold the car before the original warranty was up I'd lose that money. I forgot the exact figure they quoted me, it was somewhere in the $3600-$3800 range for one year and about twice that for two years. This is a Lamborghini extended warranty, not some third party warranty.