It seems you knew the answer before you asked the question.
The point I was making has nothing to do with any specific rule. It has to do with the fact that it is a "rule".
"Rule" 14827739 (or whatever number you want to give it): Subjects and verbs agree in number.
"Rule" 40378299 (or whatever): In forming an interrogative, do not invert the negative
not with the verb unless it is attached as a contraction to the verb.
In short, I consider
Do not you agree? ungrammatical, not just unidiomatic.
The division between ungrammatical and unidiomatic is not fixed.
In either case you end up with a structure that is not intentionally
used by native speakers. Maybe I draw the line differently than
others. I don't see why that's such a problem.
(If you want to discuss the difference between grammaticality and
idiomaticity further, you may want to start a thread in the linguistics
forum.)
CJ