The shop that built my latest tandem frame likes to set up tandems with Campy 10-speed shifters and Shimano 9-speed cassettes. As long as a Shimano rear derailleur is used, this works fine without a shiftmate. Since the cable pull on the Campy 11-speed is the same as on the 10-speed (I think), you could simply change the free hub, cassette (to Shimano 9-speed) and rear derailleur to Shimano and you'd be back in business. Or, you could go 10-speed and do the shiftmate thing.

That said, I think I'd stay with the Campy 11-speed and learn to adjust the derailleurs. If your low gear isn't low enough for you, do what you need to do to change cassettes or change chain rings. Even as an old man, I'm amazed that people are ridiculing you for starting off with a low gear that was just about a 1:1 ratio. Forty years ago when I first got a low gear so low I thought I could climb vertical walls with it. That really should be low enough for most trips, but if you need something lower, by all means go ahead and do it. IMO, you started at a reasonable low gear.