Insulation in the attic : choice between fire retardant and allergenic
TL;DR Don't worry, be happy, use the rock wool.
I am NOT an expert on insulation, so if anyone has better/more complete information, I will defer to them.
As a general rule, allergies have to do with specific organic materials - e.g., pollen - that cause an actual allergic reaction. This is a specific medical term and, as I understand it, allergies are not normally to minerals, synthetic fibers, etc. In other words, I doubt (but I could be wrong) that rock wool can actually cause an allergic reaction.
On the other hand, there are other types of irritants which can cause rashes (contact dermatitis), lung problems (asbestos being the classic example) and other symptoms, many of them quite serious, that are similar to allergies in certain respects. But they are not allergies.
My gut feeling is that none of these 3 choices (fiberglass, rock wool, synthetic insulation) will cause a true allergy, with the possible exception of outgassing of some chemicals used in production of the material. Such outgassing would be, in my opinion, a total non-issue for residents of a building if the materials are installed in a non-accessed attic. Any outgassing would escape through attic vents and dissipate into the atmosphere at very low levels.
Similarly, any particulate matter that comes out from any of these materials would be essentially indistinguishable from ordinary dust and other particles in the air from a variety of sources - soil, automobile exhaust, fireplaces, etc. In addition, properly installed insulation should have almost no loose particulate matter except as part of the initial installation. That initial installation should include cleanup of any loose particles (e.g., with a shop vacuum). The only exception would be if you used loose-fill (e.g., blown in) insulation, which would not be a good idea with these concerns, and in fact is probably best left to totally concealed spaces such as post-construction fill in external walls.
Fire is an issue. Attics get hot. Sometimes very hot. If (I haven't checked the specs) rock wool is significantly more fire retardant than other materials, use the rock wool. While the concrete should provide a lot of protection - hopefully enough for everyone to exit the building safely in the event of an attic fire - there could still be extensive (and expensive) damage. Best to avoid adding fuel to the fire.
But probably every one of these items has a Prop. 65 warning.