I've coded some simple stuff with Bimmercode: disable seatbelt chime, fold in mirrors, etc. I have not played around with expert mode and don't really want/need to.
To the best of my knowledge, yes, it backs up your last profile before coding so it can be restored.
Not sure how any of these features would "void" warranty, especially the powertrain. Typical dealer employee talk.
I bought this adapter, works wonderfully and its BT:
https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1