As an example of the classic “always ensure your software is updated” mantra, I had the latest kindlegen installed, but Scrivener was pointing to an older version of kindlegen I’d installed Somewhere Else.
The specific PNG rendering bug I showed was fixed in between those two versions.
All other points remain valid, though.
So, feel free to include PNGs in your Scrivener projects. You’ll get a nasty ugly background, but at least it’ll be one that doesn’t look like a bad frame freeze of a logo from an early episode of Soul Train.