Not sure if this is obvious: The histogram tool can be put anywhere on the palettes, and even multiple times. E. g. I have it directly above the Tone Curve tool (and below that is the HSL tool).
This is just to avoid the flipping back and forth and to have a more direct visual cue of where the right spots are on the tone curve tool.
I agree with all your points: A histogram like that as a background of the tone curve tool would be great. There is a feature request which was closed (“taken into consideration”) in December 18.