I have the Nexus 6P and the finger print scanner works great on the back. You have the option to use a pin on the screen also, so the phone laying on its back is a moot point.

Every one of your issues with Samsung would be solved if you bought a Nexus. Especially #2. No matter what I’m doing on my Nexus 6, if I get a text it drops down from the top where I can read and/or reply without leaving or navigating away from anything. Simple.. Samsung phones don’t do that?