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.

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.

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.