carpa,
1) With 32-bit float on the F6, "clipped" really is NOT. In other words, you might record a piece and the resulting file has peaks of +20 dB. In any app that handles this file format (most anything nowadays), you simply lower the level and nothing is clipped. Or if you set levels very low, you might have average levels of -60 dB. Same thing; you just raise it in post and it sounds just fine. You can set the faders really high, or really low - it makes no difference because you are just adjusting the post-ADC level. With this file format and with the dual ADCs, your dynamic range is enormous. It's a bit difficult to conceptualize until you try if for yourself.
Keep in mind that the preamp level (pre-ADC) is fixed on the F6 if set to 32-bit float mode. Again, you can only adjust the fader level (post-ADC) in this mode.
The only thing you need to worry about is overloading the analog input stage, which has been discussed at length here before. That would be the only reason to use LINE+PHANTOM, which pads the input down. I would note that WiFiJeff done classical piano recordings with the Josephson c617set, an exceptionally sensitive mic with a very hot output. If that didn't overload the F6 input stage, then I would think you would have a lot of leeway.
Bottom line - 32-bit float on the F6 is THE way to go for the way you and I are both performing and recording, as long as you plan to do some basic post work.
2) I haven't used it this way, but you can quickly play back anything you recorded. I would say that for this use, you should set it to 24-bit fixed mode so you can adjust the preamp to a normal conservative level. If you used 32-bit float mode with the fixed gain, you could easily wind up with levels way beyond 0 dBFS, and that would definitely clip your headphone out when listening back.
3) I haven't tried AC powering. If you do this, just make sure you are matching the specified input requirements. I still recommend that you go with the
Dracast 6600 mAh batteries I am using, as they last a VERY long time, and you would never need to worry about external power supply noise.
4) USB interface mode supports up to 24-bit/96 kHz. It would be very nice if a firmware update could allow 32-bit float recording over USB, but I wonder if it is a limitation of the USB 2.0 bus speed, since 192 kHz is supported when recording internally but not over USB.
5) I know this would work on Windows, and can't see why it wouldn't on a Mac. You just select your F6 as the external mic.
6) The F6 is in a completely different class than the H6 in every possible way. It is more than worth the extra $200.