The "conceivability" of zombies is accepted by a substantial fraction, possibly a majority, of academic philosophers of consciousness.

This can be made precise. According to the 2009 PhilPapers Survey (sent to all faculty at the top 89 Ph.D-granting philosophy departments in the English-speaking world as ranked by the Philosophical Gourmet Report, plus 10 high-prestige non-Anglophone departments), about 2/3 of professional philosophers of mind think zombies are conceivable, though most of these think physicalism is true anyway. Specifically, 91 of the 191 respondents (47.6%) said zombies are conceivable but not metaphysically possible; 47 (24.6%) said they were inconceivable; 35 (18.3%) said they're (conceivable and) metaphysically possible; and the other 9.4% were agnostic/undecided or rejected all three options.

Looking at professional philosophers as a whole in the relevant departments, including non-philosophers-of-mind, 35.6% say zombies are conceivable, 16% say they're inconceivable, 23.3% say they're metaphysically possible, 17% say they're undecided or insufficiently familiar with the issue (or they skipped the question), and 8.2% rejected all three options. So the average top-tier Anglophone philosopher of mind is more likely to reject zombies than is the average top-tier Anglophone philosopher. (Relatedly, 22% of philosophers of mind accept or lean toward 'non-physicalism', vs. 27% of philosophers in general.)

There is a stuff of consciousness which is not yet understood, an extraordinary super-physical stuff that visibly affects our world; and this stuff is what makes us talk about consciousness.

Chalmers' core objection to interactionism, I think, is that any particular third-person story you can tell about the causal effects of consciousness could also be told without appealing to consciousness. E.g., if you think consciousness intervenes on the physical world by sometimes spontaneously causing wavefunctions to collapse (setting aside that Chalmers and most LWers reject collapse...), you could just as easily tell a story in which wavefunctions just spontaneously collapse without any mysterious redness getting involved; or a story in which they mysteriously collapse when mysterious greenness occurs rather than redness, or when an alien color occurs.

Chalmers thinks any argument for thinking that the mysterious redness of red is causally indispensable for dualist interactionism should also allow that the mysterious redness of red is an ordinary physical property that's indispensable for physical interactions. Quoting "Moving Forward on the Problem of Consciousness":

The real "epiphenomenalism" problem, I think, does not arise from the causal closure of the physical world. Rather, it arises from the causal closure of the world! Even on an interactionist picture, there will be some broader causally closed story that explains behavior, and such a story can always be told in a way that neither includes nor implies experience. Even on the interactionist picture, we can view minds as just further nodes in the causal network, like the physical nodes, and the fact that these nodes are experiential is inessential to the causal dynamics. The basic worry arises not because experience is logically independent of physics, but because it is logically independent of causal dynamics more generally.

The interactionist has a reasonable solution to this problem, I think. Presumably, the interactionist will respond that some nodes in the causal network are experiential through and through. Even though one can tell the causal story about psychons without mentioning experience, for example, psychons are intrinsically experiential all the same. Subtract experience, and there is nothing left of the psychon but an empty place-marker in a causal network, which is arguably to say there is nothing left at all. To have real causation, one needs something to do the causing; and here, what is doing the causing is experience.

I think this solution is perfectly reasonable; but once the problem is pointed out this way, it becomes clear that the same solution will work in a causally closed physical world. Just as the interactionist postulates that some nodes in the causal network are intrinsically experiential, the "epiphenomenalist" can do the same.

This brings up a terminology-ish point:

The technical term for the belief that consciousness is there, but has no effect on the physical world, is epiphenomenalism.

Chalmers denies that he's an epiphenomenalist. Rather he says (in "Panpsychism and Panprotopsychism"):

I think that substance dualism (in its epiphenomenalist and interactionist forms) and Russellian monism (in its panpsychist and panprotopsychist forms) are the two serious contenders in the metaphysics of consciousness, at least once one has given up on standard physicalism. (I divide my own credence fairly equally between them.)

Quoting "Moving Forward" again:

Here we can exploit an idea that was set out by Bertrand Russell (1926), and which has been developed in recent years by Grover Maxwell (1978) and Michael Lockwood (1989). This is the idea that physics characterizes its basic entities only extrinsically, in terms of their causes and effects, and leaves their intrinsic nature unspecified. For everything that physics tells us about a particle, for example, it might as well just be a bundle of causal dispositions; we know nothing of the entity that carries those dispositions. The same goes for fundamental properties, such as mass and charge: ultimately, these are complex dispositional properties (to have mass is to resist acceleration in a certain way, and so on). But whenever one has a causal disposition, one can ask about the categorical basis of that disposition: that is, what is the entity that is doing the causing?

One might try to resist this question by saying that the world contains only dispositions. But this leads to a very odd view of the world indeed, with a vast amount of causation and no entities for all this causation to relate! It seems to make the fundamental properties and particles into empty placeholders, in the same way as the psychon above, and thus seems to free the world of any substance at all. It is easy to overlook this problem in the way we think about physics from day to day, given all the rich details of the mathematical structure that physical theory provides; but as Stephen Hawking (1988) has noted, physical theory says nothing about what puts the "fire" into the equations and grounds the reality that these structures describe. The idea of a world of "pure structure" or of "pure causation" has a certain attraction, but it is not at all clear that it is coherent.

So we have two questions: (1) what are the intrinsic properties underlying physical reality?; and (2) where do the intrinsic properties of experience fit into the natural order? Russell's insight, developed by Maxwell and Lockwood, is that these two questions fit with each other remarkably well. Perhaps the intrinsic properties underlying physical dispositions are themselves experiential properties, or perhaps they are some sort of proto-experiential properties that together constitute conscious experience. This way, we locate experience inside the causal network that physics describes, rather than outside it as a dangler; and we locate it in a role that one might argue urgently needed to be filled. And importantly, we do this without violating the causal closure of the physical. The causal network itself has the same shape as ever; we have just colored in its nodes.

This ideas smacks of the grandest metaphysics, of course, and I do not know that it has to be true. But if the idea is true, it lets us hold on to irreducibility and causal closure and nevertheless deny epiphenomenalism. By placing experience inside the causal network, it now carries a causal role. Indeed, fundamental experiences or proto-experiences will be the basis of causation at the lowest levels, and high-level experiences such as ours will presumably inherit causal relevance from the (proto)-experiences from which they are constituted. So we will have a much more integrated picture of the place of consciousness in the natural order.

This is also (a more honest name for) the non-physicalist view that sometimes gets called "Strawsonian physicalism." But this view seems to be exactly as vulnerable to your criticisms as traditional epiphenomenalism, because the "causal role" in question doesn't seem to be a difference-making role -- it's maybe "causal" in some metaphysical sense, but it's not causal in a Bayesian or information-theoretic sense, a sense that would allow a brain to nonrandomly update in the direction of Strawsonian physicalism / Russellian monism by computing evidence.

I'm not sure what Chalmers would say to your argument in detail, though he's responded to the terminological point about epiphenomenalism. If he thinks Russellian monism is a good response, then either I'm misunderstanding how weird Russellian monism is (in particular, how well it can do interactionism-like things), or Chalmers is misunderstanding how general your argument is. The latter is suggested by the fact that Chalmers thinks your argument weighs against epiphenomenalism but not against Russellian monism in this old LessWrong comment.

It might be worth e-mailing him this updated "Zombies" post, with this comment highlighted so that we don't get into the weeds of debating whose definition of "epiphenomenalism" is better.