1. The Windows display calibration is only compatible with narrow gamut displays, so if you are set up as a P3 screen, it should not work. Secondly you don't want two different parts of the system trying to control colour; no knowing what will happen, so shut that one down.
2. Again, strange. Rendering intents are how the display driver handles out of gamut colours. In photography we use two of the four; Relative Colorimetric and Perceptual. What Relative Colorimetric does is it moves all out of gamut colours to the nearest in gamut colour and leaves all colours that are in gamut where they are. With Perceptual, this rendering intent redistributes all colours, including the ones that are in gamut. This means you will get some colour shifts. This can be fine when printing, but can be really confusing when you are batch correcting images (and some don't look quite right).
MS at one time did not properly support Perceptual for screen displays and I don't know if they have fixed this or not.
3. Okay - I do the same thing
4. This will be part of your video card software that should be on your machine. I use an AMD Workstation card, so can't help with nVidia. Look for something that looks along the lines of this:
5. If the profiling software takes care of this, great! With the flare profiling, you'll have to test that out. I work in a fairly dark room, so not really an issue for me.
I use the same profile as the one that was output by my i1 Display Pro; in my case, I run a Adobe RGB profile to edit and output to sRGB unless I print. I work to a single setting and don't switch profiles.