This "unidentified physical reaction" would also need to not be turing-computable to have any relevance. Otherwise, you're just putting forth another zombie-world argument.

A zombie-world seems extremely improbable to have evolved naturally, (evolved creatures coincidentally speaking about their consciousness without actually being conscious), but I don't see why a zombie-world couldn't be simulated by a programmer who studied how to compute the effects of consciousness, without actually needing to have the phenomenon of consciousness itself.

The same way you don't need to have an actual solar system inside your computer, in order to compute the orbits of the planets -- but it'd be very unlikely to have accidentally computed them correctly if you hadn't studied the actual solar system.

At this point, we have no empirical reason to think that this unidentified mysterious something has any existence at all, outside of a mere intuitive feeling that it "must" be so.

Do you have any empirical reason to think that consciousness is about computation alone? To claim Occam's razor on this is far from obvious, as the only examples of consciousness (or talking about consciousness) currently concern a certain species of evolved primate with a complex brain, and some trillions of neurons, all of which have have chemical and electrical effects, they aren't just doing computations on an abstract mathematical universe sans context.

Unless you assume the whole universe is pure mathematics, so there's no difference between the simulation of a thing and the thing itself. Which means there's no difference between the mathematical model of a thing and the thing itself. Which means the map is the territory. Which means Tegmark IV.

And Tegmark IV is likewise just a possibility, not a proven thing.