Right, so your starting point is that the attacker has read-only access to ALL entropy sources, and in that scenario it's worse if the attacker has read-write access to one entropy source.

Yes. I don't find this a particularly interesting scenario, though. Sure, we can come up with stuxnet-like airgap attacks where we on-device, but not remotely, can read entropy sources. AND we can modify the output of RDRAND. And there keys have been generated for data we can later intercept. But despite that control (potentially on a CPU microcode level) we are unable to stegonographically leak it?

Sure. Possible. Has it ever happened?