After carefully thinking, I have found that most, if not all (I can't think of an exception) proper nouns can be pluralized in the right circumstances. They can also take on an indefinite article. They can be counted as well. So plural nouns are not mass nouns, nor pluralia tantum.
I still don't see an instance where a word is one and not the other.
I think we can attach articles, numbers, and pluralize all of these if we wish. They can all change throughout time, or our perception of them can change, and so we can compare two versions of them. Do you agree with this? Would you consider them mass nouns?