If this charges of atheism had merit, I would expect at least one treatise by Philodemus called Against Piety, or a polemic by Metrodorus called Against the Gods. Instead, we have the exact opposite
Your political analogy makes a lot of sense, but the matter of merit is beside the point. Many attackers neither strive to exhibit merit nor even take it into consideration. Critics see an opening, stretch a pebble of truth into a whole specious mountain, and go ad hominem on their targets. The charges of impiety and atheism were leveled against the Epicurean school. Philodemus documents it and addresses it directly in On Piety. The reason Philodemus didn't write Against Piety is because he felt that it was only the Epicureans who were practicing true piety, the only ones who had the correct perspective on the divine. Everybody else was impious. I would make the same case for Metrodorus. Epicureans could take part in the rituals and festivals with a clear conscience because internally they were practicing *true* piety and *knew* they had nothing to fear from the gods.
he left the physical details loose
I'm not sure if that's the case or not, and the physical details seem inextricably linked to their nature. I don't have Long and Sedley The Hellenistic Philosophers available right now, but I'd be interested to see how much detail there is. If I remember, Epicurus talks about the gods' anthropomorphic shape, but I've also seen scholars say that's because the gods are idealized humans, what humans may aspire to. So one has to "see" them in your minds eye as human-shaped to be able to gain inspiration from them. But Diogenes Laertius directly contrasted the Epicureans' idea of the happiness of the gods with the happiness that humans can experience:
Two sorts of happiness can be conceived, the one the highest possible, such as the gods enjoy, which cannot be augmented, the other admitting addition and subtraction of pleasures.
I also keep coming back to the emphasis and importance Epicurus placed on a correct understanding of the gods. It's first in the letter to Menoikeus. It's the first Principle Doctrine. I maintain it behoves us to examine and come to grips with this because Epicurus found this to be a foundational matter in his philosophy.