The pipeline color space is the human visible spectrum. Always. The closest RGB spaces from that are Rec2020 (which is a bit shorter) and ProPhoto RGB (which is a bit wider aka has some imaginary colors).
That’s all we care about when I talk about pipeline gamut. Visible gamut is the goal/principle, Rec2020 is the closest technological tool from that goal. Forget about Adobe RGB, sRGB and the likes. These are output media, that’s for export, not for retouching.
Then, histogram space doesn’t really matter. Just choose something that puts middle grey reasonably on the middle for legibility (that is, a non-linear color space). Anyway, it’s only a scope.
No, we don’t care. The only concern is if a color gradient looses its gradient to become a flat blob, that’s ugly. But this happens when gamut is clipped, aka the whole surface gets “rounded” to the same color. With gamut mapping, sure we will have to make some sacrifices and loose saturation, but gradients should remain gradients, although less saturated, so the image will still look believable. And 2 pixels at same hue but different saturation will still have different saturation in sRGB if they are mapped.
Just cool down. For the past 5 years or so that I have been doing opensource photography forums, people are way too concerned about gamut on a theoritical level while still producing the infamous rat-piss yellow sunsets (aka unable to see the actual gamut escapes that are right in their face while fantasizing about manual corrections they should apply to take care of gamut issues that don’t happen). Truth is gamut is handled trough intents in a semi-clever way at output, and anyway your screen most likely displays sRGB so what you see on screen is already gamut-clipped and/or gamut-mapped. If it looks good, then don’t worry.
Again, we handle gamut mapping in color calibration only to cleanup after the chromatic adaptation because we know it will push colors out of the visible gamut. It’s only intended to not make things worse and start retouching with legitimate colors in the pipe.