[Skeime]

One nitpick: Grammatically, mass nouns are not (necessarily) plural. You’d say “Paper is made from wood.”, not “Paper are made from wood.” (Of course, the pair words like scissors are different, i.e. “My scissors are in my backpack.” (I also seem to recall that there are non-pair mass nouns that only exist in plural form, but I cannot think of any examples.)

[tuatoru]

That's not a nitpick; that's agreement.

Less water, fewer onions.

Edit: not every word ending in "s" is plural. Some of them were just stolen from other languages and ended up with a final "s".

[Skeime]

You said in your post that mass nouns are “inherently plural”, which grammatically they are not. I suppose you were referring to the meanings of those words, though.