I think the confusion here is about what "fully quantum whole brain emulation" actually means.
The idea is that you have a box (probably large), within which is running a closed system calculation which is equivalent to simulating someone sitting in a room trying to write a theorem (all the way down to the quantum level). You are not interacting with the simulation, you are running the simulation. At every stage of the simulation, you have perfect information about the full density matrix of the system (i.e., the person being simulated, the room, the atoms in the person's brain, the movements of the pencil, etc.)
If you have this level of control, then you are implementing the full unitary time evolution of the system. The time evolution operator is reversible. Thus, you can just run the calculation backwards.
So, to the person in the room writing the proof, as far as they know, the photon flying from the paper hitting their eye and being registered by their brain is an irreversible interaction--they don't have complete control over their environment. But to you, the simulation runner, this action is perfectly reversible.
Now, the contention may be that this simulated person wasn't actually ever conscious during the course of this ultra-high-fidelity experiment. Answering that question either way seems to have strange philosophical implications.
What you describe is all true, however useless as described. The earlier poster wanted the simulation to output data (e.g. by writing it on paper -- the paper being outside of the simulation), and then reverse the simulation. Sorry, you can't do that. "Reversible" has very specific meaning in the context of statistical and quantum physics. Even if the computation itself can be reversed, once it has output data that property is lost. We'd no longer be talking about a reversible process, because once the computation is reversed, that output still exists.
Yet another exceptionally interesting blog post by Scott Aaronson, describing his talk at the Quantum Foundations of a Classical Universe workshop, videos of which should be posted soon. Despite the disclaimer "My talk is for entertainment purposes only; it should not be taken seriously by anyone", it raises several serious and semi-serious points about the nature of conscious experience and related paradoxes, which are generally overlooked by the philosophers, including Eliezer, because they have no relevant CS/QC expertise. For example:
Scott also suggests a model of consciousness which sort-of resolves the issues of cloning, identity and such, by introducing what he calls a "digital abstraction layer" (again, read the blog post to understand what he means by that). Our brains might be lacking such a layer and so be "fundamentally unclonable".
Another interesting observation is that you never actually kill the cat in the Schroedinger's cat experiment, for a reasonable definition of "kill".
There are several more mind-blowing insights in this "entertainment purposes" post/talk, related to the existence of p-zombies, consciousness of Boltzmann brains, the observed large-scale structure of the Universe and the "reality" of Tegmark IV.
I certainly got the humbling experience that Scott is the level above mine, and I would like to know if other people did, too.
Finally, the standard bright dilettante caveat applies: if you think up a quick objection to what an expert in the area argues, and you yourself are not such an expert, the odds are extremely heavy that this objection is either silly or has been considered and addressed by the expert already.