You're reading waaaaay too much into it. I am at least 99% confident, even after reading what you wrote, that Eliezer was not intending anything remotely like what you suggest. (I suppose there's, let's say, a 10% chance that he had some analogy between the Mirror and human minds in view. But "such-and-such an artefact, in a story about magic, has such-and-such magic powers" is not any sort of licence for assuming that everything the artefact can be analogized to has the same magic powers.

Consider: Any number of authors have written stories in which magical things stand in for the power of imagination. They aren't making a coded claim that your imagination can work magic, they are making a not-so-coded claim that your imagination can do something that is like magic. And we already know that Eliezer thinks human minds can do non-magical things that are like magic in a useful sense.

So, I think: probably (p=0.75) Eliezer didn't intend the Mirror to be a symbol of human minds generally, but if he did (p=0.25) then almost certainly (p>0.99) what he intended was something more like that "Mundane Magic" post (see the section headed "The Ultimate Power"), and in any case nothing to do with the "Law of Intention".

[EDITED to fix a trivial typo.]