In short, I'd like a way to code a difference between italics and bold that I intend for vision only, and those I intend for spoken emphasis. Is there a reliable way to do that?

Disclaimer: I know next to nothing about screen readers.

But if it's true that they ignore most CSS, you could use that: <span style="font-style: italic">italic text goes here</span> (or <div> instead of <span> if you want it to apply to one or more paragraphs rather than to just part of a paragraph).