I updated my 8100 without issue. Some answers to your questions:
1.AFAIK, avicsync is still not working...at least not for android 6.0.1. My app won't display the maps, and I also don't see much value to the app itself, apart from being able to program multi-stop routes from my phone. Even then, this functionality overlaps with other mapping apps, so the net-net isn't much, IMO.
2. My settings all carried over after the update.
3. I didn't have to reinstall any map files, but I haven't updated anything either, so can't comment.
4. I didn't update any Bluetooth components,