Hi, rhino. Good to hear from you.
You say, "there would definitely not be a single form of the word". That's true, but that doesn't mean there is no word
mump. There is, with a number of meanings, none directly related to the disease, and
mump is an accepted word in Chi. That's why
mumps is not allowed at present.
It does seem to be a fairly similar situation to
measles, which was discussed
here.
Measles is allowed as a common word, while
measle is accepted as a rare word. Maybe this is the way to go with
mump/
mumps.