FWIW, here is a link to a related thread concerning your crankset:
http://www.bikeforums.net/showthread...a-Tandem-crank

Since your CoMo is a 2012 model, likely you are running a 10spd config?

I would not look at the cranks themselves as playing much of a roll in your downshift->granny problems, but it could be related to chainline, ie: your driveside may be setup too far to the inside, an issue which could be resolved by using BB or axle/spindle spacers. Next, your cranks likely came OOTB setup for 9spd, which typically isn't much of a problem, but maybe that depends on the quality of the chainrings provided with those cranks. You may want to look at replacing your chainrings... not all are created equal. The best bet for 3rd party 5-arm rings is: 1) Shimano, 2) Specialites TA (go to Wiggle.com for these), 3) FSA N10 rings.

Other than chainrings, it could be your FD simply has a weak return spring. Perhaps have the shop let you test out a new triple FD.

Chain mfr & model? I have had great shifting performance and durability using Shimano Ultegra 10spd chains on our tandems.

Hope that helps.