If you want to go down the physics route translated into a modern paradigm, I'm going to posit that the intermundia refers to another dimension or another universe in the multiplicity of universes out in The All. Gods are material and natural, but reside outside our universe under a different physics than our own. We cannot see them with our eyes, because our universe doesn't overlap with theirs. They are not aliens living on planets in our universe. That's just an assertion on my part, granted. But that's the only way I could right now "accept" a corporeal divinity in an Epicurean theological context.
I'd say your suggestions there are possibilities, but not really compelled at all by the texts. I would expect Epicurus thought of the universe as "all that exists."
I find it much easier to think of the gods as what an ideal life would be like without the limits on mortal, corruptible bodies.
I think this indicates our difference in perspectives. I have no problem stipulating that it is possible within natural physics for a living being to find ways to completely replace its own atomic structure over time so as to not be subject to the necessity of death. In fact I'd find it much more difficult for me to argue that it's "impossible." Absent the standard possibilities of humanity blowing itself up etc., I'd wager we are me no more than a couple of hundred years from that ability ourselves, at the outside.