So is macaroni & cheese "mac" because it's short for macaroni... Or is it because "mac" is an acronym for macaroni & cheese?

I don't think so. Mac and cheese would be the correct usage.
If I walked into a pasta restaurant and asked for mac they would ask a further question to clarify.
If I was to say mac and cheese no other question would be needed. 😋

I always hear mac and cheese