Paul, Dietz, some more info to help with the hunt. After further testing and investigation I now start to think that the two issues (MIR not loading the plugin info and the audio disappearing from my RME interface) *might* be unrelated.
MIR not loading the plugin info is consistent and repeatable, I have no doubts about that.
However, it seems that there might be an issue with the way the iMac's USB3 ports handle USB2 devices, e.g. RME's interface (or even an iLok key). I've posted about it in the RME forum and others have desribed weird issues as well.
As of now, I can open Cubase sessions with MIR loaded. I have to do it twice, of course, because of the bug originally described. When the audio disappears, I power cycle the RME while Cubase is running. Cubase complains and switches to Built-In Audio. I then switch back to RME and can work again.
Looks like two distinct bugs now. I hope this helps.