The IRM differs from the IRB in that the IRB is one shader with controls for 20 different formations and a number of effects and the IRM is 18 individual shaders with default formations that differ from the 20 in The IRB. The IRB is to create your own formations by mixing and experimenting, the IRM is ready made shaders of individual formations. They each control mostly the same, each have their differences, but both are for the same purpose of building procedural rock formations.

Yes and no. Eevee can't handle the complex shaders or material displacement, so switching to Eevee with any of the shaders in the scene may result in a crash. However, using the baking controls, you can bake out your formations into physical geometry, at which point switching to Eevee will be fine.