>Since we are working with a monospace font, every glyph must have the same advance

Just pointing out that this is merely an expectation and not a rule. You can happily design a "monospace" font in any font publishing software that blatantly disrespects a single width. In fact, you can just manually make any letterform 2x width, and that is exactly what most monospace ligatures do! (except the bad ones, which combine e.g. "fi" ligature a single width letter)

The ligature letters are just... 2x width. Believe it or not this is totally legal!

https://www.hanselman.com/blog/monospaced-programming-fonts-...