1902 house. Really cold winters. Insulation question
The only issue I can see is that the rock wool is not sturdy which wouldn't give you anything to anchor your new siding too without building some sort of frame to fasten your siding to.

Now, what we (my parents and I) did on a 1903 farmhouse in Saskatchewan was to use that "rigid insulation" and fastened it to the stucco (concrete if you will). We then covered it vinyl siding, but I don't see any issue with going with cedar siding. This has held up just fine for about 8 years now.