Grant: this is a very detailed checklist and I appreciate the effort and the advice. I'll make sure I follow it.
I'd posted a reply to the original posting so I thought I'd update people on where I am in all of this.
I have two external monitors that I connect to my 13" early 2011 MBP. (Yes, I use an Apple adapter.) One is a small NEC; the other is a large Samsung plasma. The problems had been with the Samsung.
As I have a work around that gets the large Samsung external monitor to work, I'd stopped fretting about this.
Still, I had been convinced that there was something wrong with the Thunderbolt firmware, since the malfunction of "Detect Displays" did not occur till after the September 2011 firmware update.
Since that time, clicking on "Detect Displays" (whether on the Menu Bar display, or within System Preferences itself), produced no effect. Nothing got detected. I had to use workarounds every time I swapped the external monitors.
Just this past week, there was a new firmware update. I held my breath as I installed it. I'd booted up with the NEC monitor attached and "mirroring" off. Everything worked perfectly and automatically. Then Wednesday night, I unplugged the NEC and, without rebooting, plugged in the Samsung for the first time. I was "mirroring" the laptop and the external screen. Everything worked just like it used to. I was so pleased.
Then, after finishing my presentation on the Samsung, I swapped out the Samsung and, without rebooting, or turning off "mirroring," I plugged in the NEC. The MBP knew there was an external monitor attached, but it could not connect at the correct resolution or frequency. Even worse--since I'd been "mirroring" on the Samsung--I was still set on "mirroring" when I plugged in the NEC. The screen on the MBP went as whacky as the screen on the NEC: both were a pale blue with no desktop, icons, etc. I had to reboot the MBP. The external monitor could not be detected. So I rebooted again, holding down two keys that AppleCare had told me to hold down to reset the powermanagement controls for the Thunderbolt port. Once it was rebooted, I was able to plug in the NEC and it was detected just fine. Since then, I've only had the NEC plugged in so there's been no need to fiddle with any more of this. But come Wednesday, I'll need to make my weekly presentation with the Samsung, so it'll be interesting to see what happens.
So, it seems to me that there's been some improvement given the recent firmware update, but it's still not perfect. Next week, when I swap in and out the 2 LCDs, I'll make sure that "mirroring" is off before I make the switch back from the Samsung to the NEC. Hopefully, with mirroring turned off, the NEC will be detected at the correct frequency and resolution and everything won't go whacky. Otherwise I don't know what to do other than restart the MBP before plugging the NEC back in. It sure would be an annoyance to have to reboot but if that's a necessary work around, well, I'll work around it.
Maybe if enough other people keep having problems with this, a third firmware update will put this problem to rest.