Since the word "who" can be used in both singular and plural forms (as in "Who are your friends"), why is it incorrect to use "are" in the sentence " Who of us are more?" which is about distinguishing a group of people from another group of people?

Question

Answer

I think the issue is with 'who' rather than 'is' or 'are'. We generally use 'which' in this structure. The rest of it is as you say.

Which of us is going through to the next round?

Which of us are going through to the next round.

Answer

It is not incorrect.

Although the sentence "Who of us are more?" is correct, it would sound better if you used a different preposition to emphasize the plural sense of "who":

"Who among us are more?"

You could also use "whom", but you do not need to.

Answer

I would say it's not incorrect, it just sounds weird because people have developed a habit, in that type of sentence, to use "is" instead of "are". I can not think of any logic explaining it, other than it's a habit that people have become accustomed to.