Hi,
1 - Yes you can roll back, you need to download the 'Windows Device Recovery Tool' on a pc/laptop. It's straight forward to use.
2 - It is possible, but it hasn't personally happened to me (*touch wood). Even if you bricked the phone, it's still possible to recover it most times.
3 - As far as I know the Lumia 920 and 925 are decent devices and Win10M should run ok. I ran earlier builds on a Lumia 625 (512MB) and found it too slow. You would probably be better off checking out the 920/925 forums to get a better idea from users who have actually run Win10M on those devices.
In my opinion, you should be fine with the latest build (.164) on your devices.