Thanks for your response. Through this post I’ve learned a lot more (somewhat obvious, in retrospect) ways that this can be used, and how the tech has simply failed us all. You’ve actually taught me a few more things I hadn’t considered: OS differences and more options that I’ll need to be aware of.
In my software, I’ve added shortcuts for page size, too, and it seems to override the ability to select it from the browser side (letter, a4, and legal were the most prominent, I felt), which lowers the number of options for users. Setting page margins does override browser settings as well, but you’ve read this, so you know how well that worked out.
Thanks for the spec link. It seems as though what I’m doing is technically valid, it’s just being handled incorrectly by each browser. It does show more options that I was semi-aware of (MDN is even lacking in description for most of this, which is sad), so maybe there is more I can do.