Sorry for the triple post but I have good news to report! I rolled back to Nodo and tried again, but I did 2 things differently this time before installing Mango again. First, I uninstalled the HTC connection setup app. Second, I made sure to turn cellular data back on after doing the "pull the plug" trick to get the update to show up but BEFORE I actually installed the update.
I'm not sure which one of these helped but I now have working 3G on Mango! I haven't reinstalled Connection Setup for fear of screwing things up, but I never really needed it anyway.
Hope this helps.