Let me just touch on this: Adobe RGB is not the “maximum” color space. From Wikipedia: " When defining a color space, the usual reference standard is the CIELAB or CIEXYZ color spaces, which were specifically designed to encompass all colors the average human can see."
ProPhoto is a smaller color space (but bigger than Adobe RGB) and Lightroom uses a custom variant of this color space for all internal work. I hope that PL does not use Adobe RGB for its internal color space.
As far as I can tell, this is incorrect. The histogram is based on the color profile chosen in the preferences dialog, not the camera’s JPG rendering. Cameras can often render the same RAW file in several different ways (using “creative” modes), which is what I believe PL’s Color Rendering tool also does. These rendering modes alter the color—color space conversions, on the other hand, try to maintain a color even as the numbers that represent that color change.
As a wild guess, if you want the color as captured by the camera’s sensors, you need to have a profiled monitor, you need to make sure PL is using that color profile, and you probably need to have Color Rendering disabled (or maybe set to Generic Renderings/Camera Default Rendering).
Since individual sensors may be slightly off, you can use something like the X-Rite color chart and software to create a DCP (or is it ICC) that will correct for that specific sensor. The default is for a “typical” sensor for your camera, which isn’t always right.