I played around with it last night, and I think I'm missing something with the TMing... my results were ok, but the two cab version still sounded much thicker even after I compensated for the volume loss when using just one cab.
I'm convinced at this point that has to do with me being a TM noob, and I still think your idea is solid, but I think we might be saying the same thing in different ways regarding the cab block settings? When using the TM'd UR IR made from the two cabs going into the TM block with my preset, I had all of the settings in the cab block as flat as possible, and tried with both "none" and "null" mic models. The sounds weren't "bad" by any means, but were still noticeably different from the two-cab version. I wonder if it wouldn't have something to do with the way the source & local signals are panned hard L/R?
BTW, I also tried your idea on the shimmer patch. Works like a charm! It wasn't quite where I want it to be, but that's 100% because I need to go back and dial in the one reverb block correctly to replace the two (obviously, a lot more going on with a 2nd independent reverb).