1- what sort of warranty would you want invalidated? it's just a bodykit! replacement panels, that's it! They'd had to prove whatever goes wrong with your car, is directly related to the bodykit... for more info have a nose into the "modifications section" there's a thread about warranty there http://www.350z-uk.com/forum/viewtopic.php?t=326
2- I didn't have mine resprayed, and it depends who takes it off in the first place, the oem spoiler is glued down quite hard, but it can be done without damaging the boot...
Also, the Nismo spoiler will require 2 extra holes to be done on the boot, so there's no going back