Yes they are. And because of that, you should not hit (above) 0dBfs.
Yes they do. And analog stage of properly designed A/D converter should be operating (relatively) linear manner up to 0dBfs.
Yes you are. They definitely are not. They have inferior S/N, distortion, jitter, etc figures. But even with them there’s no need to record at low levels. Take a look at http://www.youtube.com/watch?v=BYTlN6wjcvQ (starting at 41:15). (uncompressed audio files available at http://www.ethanwiner.com/aes/)
So do I. But being a pro-sumer doesn’t imply: you’ll have to record below -6dBfs.
No. And it really doesn’t matter. If you’ll have straight chain: Microphone -> Integrated preamp+A/D+interface, you don’t need to know, at which voltage levels the signals are there inside the box.
Some do and some don’t. Well-coded floating-point plugin behaves exatly the same at any levels, if not coded to behave other way on purpose.
Nothing wrong with that. If it suits your workflow, you should do it like that. But your way is not The Whole Truth And Nothing But The Truth. It’s your way. My way is just play everything out from the tracks, no matter what level they are and adjust levels with faders. And use master fader to set the final level. Only if inserting something analog on the path, I may use digital trims in my console to prevent D/A from clipping. But my way isn’t either The Whole Truth. We all have our own working methods.