Well for one thing, any ceramic or other durable remnants of a civilization prior to ours would only have had to have lasted for several hundred million years a

Well for one thing, any ceramic or other durable remnants of a civilization prior to ours would only have had to have lasted for several hundred million years at most, not billions, since before several hundred million years ago life of such complexity would have been vanishingly unlikely or impossible. Secondly, for any prior civilization, the whole sun and cinder thing doesn't apply anyhow and finally, if we or some civilization before ours had vanished, at least some of our durable goods would be conserved in some sort of preserving environment. As another reply here mentions, we have wonderful fossils of some extremely delicate creatures from deep history, a toilet or two would hardly need much to pull off the same degree of conservation and later be discovered. Your notion of it being extraordinarily unlikely doesn't really fit existing circumstances.