This is an answer based upon observable grammatical facts, but not drawn from any published, vetted grammar reference.
Verbs must agree with pronouns occurring as subjects or within subjects:
- He is lucky.
- He who is without blame is lucky.
- Her student couldn't attend. They were otherwise engaged.
In (1) and (2) we see the verb be agreeing with the pronoun he. In (3) we see that the pronoun they, as the subject, commands plural verb agreement despite the fact that it is being used to refer to a single person.
However, in NPs with common or proper nouns occurring as the head of the nominal phrase, the verb agrees with that noun in terms of number, and is always third person if the noun is singular. It does not matter who the noun refers to:
- Araucaria is writing you an answer.
- *Araucaria am writing you an answer.
In (4-5) we see that although Araucaria refers to me, the writer, the verb be must take a third person singular form, not a first person one. (5) is ungrammatical.
Now, even with the verb be, which is more highly inflected than any other English verb, the form of the verb is identical for the first person, second person and third person plural:
- we are happy
- you both are happy
- they are happy
And, as with the pronoun they, it does not matter if the pronoun you is used to refer to a single person or multiple people:
- you, my friend, are happy
Arguably, an English verb table for be should look something like this:
1st person singular: am
3rd person singular: is
Plural: are
And it should be understood that you in English is always grammatically plural (in terms of grammatical number, not semantics).
The Original Poster's question
The Original Poster asks why we can say both you idiots (plural noun), and you idiot (singular noun), but only the first of these can occur as a subject.
We noted two points further above. Firstly, a verb must agree with a pronoun in a subject NP. Secondly, an NP with a singular common or proper noun as head must take a third person singular form of the verb.
With the NP you idiots these rules don't present a problem:
- You idiots are unbelievable.
However, with the singular you idiot we have a problem. The pronoun you means that only a plural form of the verb is permitted, but the singular common noun means that only a singular form can be used. The result is that whether we use a singular or plural form of the verb, the result is ungrammatical:
- *You idiot is unbelievable.
- *You idiot are unbelievable.
I deeply suspect that this is at the heart of the problem with using you idiot as a subject. However, as Pullum always says: with regard to English grammar, the truth is rarely pure and never simple. My reason for saying this is that you idiot is not available as a subject for a non-finite clause either:
- It was necessary for you idiots to win the tournament.
- *It was necessary for you idiot to win the tournament.
Such is life ...