&learningjam2024
Recently finished reading through the paper [PM] I mentioned in my last post. It certainly clarified some things and my summary has some inaccuracies in it.
Until I get around to address that I'd like to mention that equation (3), which I got from the Disney paper [DS], also has to be multiplied with a "normalization" term which is shown at the top of page 25 (eq. 3, the left fraction on the right side). This wasn't made clear to me until it was mentioned in [PM].
This is my usual experience with reading papers of this kind. Often I need multiple sources in order to fill in the gaps of my understanding.

Testing in my Shadertoy demo, the specular highlight (D-term) now behaves more or less as I would expect. Still got aliasing at the penumbra which appears to be due to the G-term (which handles micro facet shadowing), so that's what I'll be working on next.