The plural elohim does nothing for polytheism, when the singular is used with the plural to demand the use of the majestic plural.
You're assuming that everyone reads it as a majestic plural. Some don't. They read it as a simple plurality of persons/Gods.
The "us" "our" use shows only that the elohim is talking to one other than that elohim, so the same would be true.
The same is not always true, because it is no stretch to consider that the Elohim speaking might be talking to another Elohim, since man is made in "their" image. Can you see how some can "do as much" now? There's more than one way to cut a radish.