[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.