The computational theory of mind (CTM), multiple realisability thesis, and the question of whether abstract entities are Platonic entities or something else. These are all well-worn topics in analytic philosophy. In my PhD in the philosophy of information I researched the possibility of information being 'Platonic'. (Which possibility I rejected.)
The multiple realisability problem - which applies to both to computation and cognition is what your cited RS paper is referring to.
You deploy a lot of basic non-sequiturs. For example - you don't get to panpsychism from multiple realisability of consciousness. That doesn't logically-necessarily, nor naturally-necessarily, follow. There are several alternative explanations that satisfy IBE (inference to the best explanation) better.
Maybe there is no more to consciousness than computation. Certainly, however, science knows of no examples of conscious awareness in the absence of complex physical information processing by naturally-necessary physical information processes.
There's no evidence at all that your granite slab is anything like conscious. The most remarkable thing it doesn't have is the right kind of complex information processing systems. Panpsychism is a game some philosophers play because they're bored and want to sell books. Like most dubious philosophical doctrines it has roots in religion and mysticism.
