Dire Bare
Legend
So I just cast Commune and ask "is Zeus a demon?" and then "is Zeus a devil?" and then "is Zeus chaotic good" and then I probably get a few more questions left over. In that way I've saved thousands of years of misunderstanding. Or I could plane shift to Olympus and ask around. And what cause would a monotheistic religion have to lie about this? Even an evil one? Why would they care what Zeus is? A monotheistic Chaotic Evil religion might say "yea, the other gods are good, big deal, worship with us or die" A monotheistic Lawful Good deity would gain what by insulting the other good deities?
I'm not saying what you're saying can't be made to work, I just would find it very difficult to run in a campaign subject to a certain level of scrutiny.
If my priest of the one true god communes . . . who is answering his spell? His deity and/or deity's servants. And they will most likely answer, "Oh yeah, that Zeus guy is a total demon." And how many characters in the world can even cast spells like commune, or better yet, planeshift. And if you do planeshift to Zeus' pad, and you believe him to be a deceptive demon . . . OF COURSE his servants are going to "lie" and claim he is benevolent. And they might not be too happy to see you, a priest of a god who claims their divine lord is a demon!
Why would a "good" religion lie about the metaphysical truths of the setting? Why do "good" religions of the real world demonize "good" deities from other competing religions? If it happens in the real world, shouldn't be hard to make it happen in your fantasy world.
I just don't see any difficulty whatsoever to use a monotheistic religion, good or evil, in your campaign world.