Would get in touch with the 'tana folks and see it they have a solution to your rim/tire problems. Seems the bike shop mech knew what he was talking about!
We run folding tires (Maxxis Re-Fuse 700x25) on Velocity Aerohead rims for 22,000+ miles and never had a tire blow off. I can install/remove tires without tire tools so they are not a hard fit. However, we do use Veloplugs instead of rimtape which gives a tad more room for installing tube/tire.
Usually when we install a new tube in tire, we inflate it with a couple pump strokes and then deflate it. Only after that do we inflate tire to the max, eyeballing the rim to see that tire stays properly seated.
In our decades of tandeming have had 3 tires blow of a rim while riding. One was a Contintental tire (back when they first came out) and the other 2 were tires sent by Avocet for us to test ride. Both Avocets blew off the rim . . . one front and one back, in less than a hundred miles.
Some tire/rim combos seem to be more prone to problems.
BTW, we do not use sealant in our tubes; tried it and felt it was too messy when you did have a puncture that would not seal properly.
Be thankful the blowups did not cause you any worse problems.
Pedal on TWOgether!
Rudy and Kay/zonatandem