M does happen to be the Roman numeral for “thousand” — as seen in the credits of old movies. But let’s not go there.

k = thousand, M = million, G = billion

There is actually a standard for this. I'm fine with MM, it's confusing to me but not ambiguous, just please don't reassign existing prefixes...

Source? Every source I've seen says that MM is derived from the Roman numeral M, meaning thousand.

Wouldn't that make MM 2,000, the same way II is 2 and XX is 20?

It would be if it was interpreted as an actual Roman number. But in this case it's treated as M x M.

The actual Roman version of a million is an M with a bar over it, where the bar means x1000. But that's not an ordinary character, so wouldn't work for this purpose.