Yes, its fairly easy to unplug the dynavin and temporarily plug the stock head unit back in if needed. If you go with ANY aftermarket headunit in an E9X you definitely want to keep the stock head unit handy for this reason.
Now with that said, ive had an aftermarket headunit in my car for 2 years..... haven't needed to pull it out yet....and Ive only seen a handful of people mention it on here. It doesnt worry me but Im an installer so putting the stock headunit back in for a day is not a big deal. If it is a big deal to you just do the relocation thing then it wont be an issue anyway.
you shouldnt have any trouble with the warranty, you would only have trouble if the aftermarket stereo or install caused the problem that you are trying to get warrantied. So long as you are competent, dont do anything really stupid, and install everything correctly this should never be an issue. This is not headunit specific, it goes for anything aftermarket you might install on your car.
Last edited by jeffb335; 03-11-2013 at 01:22 PM.