Makes sense. Here is an updated patch with this change.
I didn't end up renaming anynullkeys. I came up with names like "anyalwaysdistinctkeys", but in the end that felt too abstract, and moreover, it would require rewriting a bunch of code comments that refer to null values in this context. Since as you wrote, anynullkeys is just a local concern between two functions, this slight inaccuracy is perhaps better than some highly general but unclear terminology.
Agree with that. With the comment it is clear how it works.
I've looked at the patch v3. It seems good enough for me. CFbot tests have also come green.