I have some more data for you on the dongle with boot commands. Turns out I was right - at one time I was able to reboot my iMac, pressing the option chord, and it worked.
I think most of the time when I was testing recently, I was selecting "restart" and holding "option" as soon as I heard the startup tone. And it failed to do anything every time.
Anyway, I just got done another series of tests. Restart still didn't work. But when I was connected to the dongle and did a shutdown, it worked every time (only 3 attempts, but 100% success).
So, it would seem there is a difference in what the iMac does on starting again from a restart and starting after a shutdown.
Hopefully, this will always work since, if I'm really stuck and need this option, everything would be starting after a shutdown anyway.
Now we need some other testers to run the same test:
Be sure you are connected with the Dongle.
After the screen goes off, press the power button to start again.
When you hear the startup tone, hold the "option" chord until you get to the screen where you can choose a boot device (I have no idea what would happen if you have no other devices connected that you can boot from though!).
It would not work on BT, but I noticed something different about it. I was holding the right option chord on these tests. Normally that lights up the 7th and 8th LED. But when doing this with the BT connection, while those same two lit up, the 6th LED would slowly go on and off.
Now that I've solved all Apple's flaws, tell them to send me a maxed out iMac Pro. That will hold be until they ALSO ship me the new Mac Pro when it's released.