But a dominant 7th chord doesn’t have a minor 7th. It has a flat 7th. You might call that semantics. But there’s nothing “minor” about a V7 chord. Nobody would ever call a C7 a CMm7. Of all your permutations of this double m notation, the only one that actually exists is the mM7, at least as far as I’ve ever seen in my 50 years playing and reading music.
The symbol for a major triad is the root alone, no M suffix. Add an “m” or “min” or “-“ for a minor triad. A dominant 7, just adds “7”, not “Mm7”. I’d still be interested to see an actual example of this Mm, MM or mm being used.