I was attempting to translate a sentence about the weather and I was told that "minus" was not declined...in any case. I also heard, though I don't know, that this is also common in mathematics. (Ignoring the singular/plural distinction between the two languages)
At night, temperatures range between minus ten and zero degrees Celsius. Но́чью температу́ра коле́блется ме́жду ми́нус/ми́нусами девятью́ и нулём гра́дусов.
Is there a particular reason I was told this (even an obvious one I am overlooking...)?
As a preposition (ме́жду), it seems to work with instrumental, and rarely, with genitive (wiktionary).
My confusion lies in why the nominative (maybe accusative) is used. I think after the preposition it should be instr. or gen., as alluded to in the PS.
Additionally, 'minuses' might seem logical at first, but then considering all the times when it doesn't follow someone's logic - there are many of them, e.g. for cardinal numbers:
1+ Nominative 2-4 + Genitive singular 5-20 + Genitive plural
Of course here I overlooked that the cardinal number is singular... But your explanation just doesn't address the case.