Hi Steve,
Great intro. A couple of things that you might want to check. AFAIK DCI-P3 doesn't define a fixed white point. It can use D65, but also D55 and other values.
Re your colour volume graph, I know you use BT2020 and BT709 as a shortcut, but it would be clearer (less potentially confusing) to label these BT2020 HDR and Rec-709 SDR.
It's perfectly possible to display UHD Bluray in SDR BT2020 (I would even recommend it with projectors that can't reach more than 300nits peak brightness in HDR on the smallest screens and much less on larger screens, and even then have to seriously raise the black levels to achieve such highlights in HDR, unlike OLED which can only reach 500-600nits but keep their black levels). In that case (SDR BT2020), the peak brightness is no more than 100nits (most likely around 50nits if calibrated for a dark room), just like for rec-709 SDR. BT2020 is only the gamut. It's only if you add HDR that the volume increases. Although with Rec-709 we didn't have to make this distinction between SDR and HDR, I think it's going to be quite crucial to make it for BT2020, especially until HDR10 becomes more of a standard (the end user part isn't defined and left to manufacturers, which means it's almost impossible to calibrate an HDR10 display accurately unless the manufacturer provides some form of proprietary autocal or provides the golden reference for the display to the calibration software, like Dolby Vision does).
By the way all the first UHD Bluray titles are mastered in HDR BT2020, but SDR BT2020 is part of the standard and we might see titles mastered that way as well at some point. So this distinction is not only valid for those who prefer to convert HDR to SDR BT2020 due to their display limitations.
Finally it would also help to clarify that the 10,000nits is the theoretical maximum for HDR. You do this with BT2020 explaining that most consumer displays can't reach the limits of the container yet and even studios use displays with a native gamut closer to DCI-P3 for grading, but the same applies brightness wise for HDR. Because most consumer displays struggle to reach even 1000nits (on a small window, not on the whole screen), most content is graded today at 1000-4000nits, so there is space to grow there as well.
If your last graph displayed the actual (not theoretical) color volume used both on the mastering side and on the consumer display side for UHD Bluray currently mastered in HDR BT2020, it wouldn't go higher than the 1000nits mark in practice at the moment (because while some titles are mastered to 4000nits, no consumer UHD display can reproduce this, even the Dolby Pulsar grading monitor that goes that high has to use 1080p panels and water cooling to achieve 4000nits).
You're much better than I am at explaining things in layman's term, so that's possibly why you left these details out.