Allow me to echo your sentiments with regard to the job Rose did with this breakdown. Really well done.
Re boldface: When you’re Damien Chazelle, you don’t worry about any of that because you are an already established writer-director.
If not at that level, even though it’s a small concern — boldface or not — it still is a concern because you are right: some readers find it distracting.
To me, it comes down to two considerations:
(1) Why would I use boldface? The obvious reason is to draw the reader’s attention to what I’m bolding. An object, an action. If I were to feel that THIS MOMENT is super important and I want to make sure the reader actually READS it, as opposed to glossing over yet more scene description, maybe I decide to go the boldface route.
(2) What is my story’s Narrative Voice? As I like to say, “You may be WRITING the story, but who’s TELLING it?” That’s the invisible character in the story what I call Narrative Voice. Here’s a formula for that:
Narrative Voice = Genre + Style
Is boldface appropriate / natural for the story’s Narrative Voice?
A writer should feel free to use boldface, there’s no rule against it, however, it’s wise to use discretion so as not to overburden pages with lots and lots of boldface content.