Okay, I'm still having problems.
First, I uninstalled MPR++ from UM5, and I continued to get the freeze-ups & CTD's (which I suspected might happen). So then I uninstalled UM5 as well, and then reinstalled UM5 using the 2.4 version. However, I'm continuing to experience CTD's and freezes, with all five factions this time around (apparently I really was just lucky before that it only happened with the Klingons & Romulans).
Yet other mods work just fine, including BOP and UDM3 with MPR++ installed. And I didn't have stability problems playing UM5 with the 2.4 update prior to installing the MPR++ add-on, so I don't understand why I'd be having issues now. Any ideas?