These rules require the consent of the "parties", plural to waive a jury. The right to a jury trial does not mean the right to waive. Again, I could be wrong, but this is the plain language of the rule.
You may be right, that once both parties have demanded a jury, both have to consent to change it to a bench trial. That makes sense, IF, the rule in NC so provides. Rule 39 (a)(1) does seem to say that.
I would like to hear from someone who knows the practice and the law in NC. I really don't, and for the law we find, there's all the law we haven't seen yet.