> It is ngh, low rising tone. I'm sure you can verify this online.

There is no official romanization of Cantonese. Both the negative prefix `mh` and 5 are bilabial nasal, not velar nasal. They are of different tones though. Ans also as I pointed out earlier, there are velar nasals in Cantonese. to use your own logic:

> languages are generally not defined by how they are written, but by how they are spoken

So here is my understanding: one nasal that is never used in isolation is not a word.