Emergent from particular computation ≠ emergent from arbitrary computation. Plus, given your “viewed the right way” caveat, it's hard to argue that those “consciousnesses” (to concede that point, for the sake of argument) even exist in this universe, rather than existing purely within the concept-space of all mathematical entities.

Greg Egan's novel Permutation City is an excellent read for those who wish to read scifi based on this very premise.

See also Anathem by Neal Stephenson.

Yes they could, why not? From the perspective of the consciousness being created from the rocks computing, it doesn't necessarily need to know what the material is of its hardware. ie let's say we were in a simulation and the layer above us were using rocks to compute, we wouldn't even know that that is what they are using.