Referring to the monotheistic God is no different than referring to the polythiestic Zeus. In both cases you are using a NAME
You're worse than I thought. For someone who claimed to have edited grammar books, you sure are ignorant of this subject. Either that or you've realized your mistake but is unwilling to admit it.
Even elementary school level knowledge of proper English grammar knows that an adjective modifies a noun. Of course, proper nouns are modified differently from common nouns in the sense that it's not the entity that is being modified, instead it's the entity's name.
So if we are to look at those two cases:
"the monotheistic God"
monotheistic = adjective
God = proper noun
This means that "God" is being modified by monotheistic. So if it's grammatically correct, you are saying that there are more than one god named, God. And you're just referring to the monotheistic version of God among other Gods out there.
This is illogical because it's a contradiction. The idea of the existence of more than one god with the name, God contradicts monotheism, therefore the god that you're referring to cannot be monotheistic god.
"the polytheistic Zeus"
polytheistic = adjective
Zeus = proper noun
This means that "Zeus" is being modified by polytheistic. So if it's grammatically correct, you are saying that there are more than one god named, Zeus. And you're just referring to the polytheistic version of Zeus among other Zeus out there.
This would mean that all of the gods named, Zeus are polytheistic. This means that "polytheistic" cannot be the differential factor that distinguishes this Zeus from the many other Zeus that exist.
Sounds like nonsense? Well, it's only going off of your argument using proper English grammar.
BTW,
If you are using "the" then you've made a grammatical error. The proper English grammar would be to do this:
Saying that the Monotheistic God is equivalent to the Polytheistic Zeus because monotheistic and polytheistic are part of those two gods' name, and they're not being used to describe the two gods..