For Persian I refer to the writting of Antonio Panaino who is iranologist and wrote about this in 1998. Murray's book is 1913. Panaino explanation are clear and well informed, he is a recognized scholar on that.
What has he said on the subject, and where has he said it?
For rocca in Italian, one would have to look at the italian etymology. I don't know if rocca was meaning fortress in an Italian dialect (which one?) by the end of the 10th century? Rocca, roch, roche, roca, rock in romance languages and in English (by French influence) are all connected to the root of rock, meaning a big stone. What is true is that the Arabic word of "rukh" or "rokh" used at chess had been understood as roca/rocca/roch/rock etc. in Western European languages when it entered in those lands. This was natural by phonetics. And the representation of a solid rock by a castle/tower was also natural and it happened in many places in Europe as you can see on http://history.chess.free.fr/first-european.htm
Since your images portray pieces that look like fortresses, this supports what I was saying, which comes from Forbes and is agreed with by Davidson, that rukh happened to sound similar to rocca, an Italian word for fortress. It's certainly possible that rocca is related to the word rock, as rock was probably a common building material for fortresses, but without a related word like this, I don't think that sounding like words meaning rock would have led to representing the piece as a fortress of some kind.
What has he said on the subject, and where has he said it?
Since your images portray pieces that look like fortresses, this supports what I was saying, which comes from Forbes and is agreed with by Davidson, that rukh happened to sound similar to rocca, an Italian word for fortress. It's certainly possible that rocca is related to the word rock, as rock was probably a common building material for fortresses, but without a related word like this, I don't think that sounding like words meaning rock would have led to representing the piece as a fortress of some kind.