I believe that this saga is finally over!
I saw George today, who with considerable effort managed to get Junction-A secured beneath the stem. Even from several feet away I heard the reassuring click. It took more effort than I was comfortable applying. The last time I did something like that I actually pulled off the band. If I had broken it again, I would have wound up right back at the bike shop. To cover any eventuality, I figured it was better to start at the bike shop.
I'm probably jinxing myself by saying so, but I can't imagine what else could go wrong. I'll probably throw the bike on the trainer tonight so that at least I can run through the gears. I'd still like to configure the wireless unit so that I can connect to the system using my mobile phone, but there's no urgency there. Eventually I'll take a look at the synchronized shifting modes, which I can enable from Junction-A.