As for the rear brake
No defective (or damaged) parts found, so no basis for a warranty claim with Tektro. The mechanic's seen one other SL 1 with the same recurring rear brake problem (one piston sticking against the rotor).
He's thinking now that the magnetic rear wheel sensor has magnetized one of the steel pistons, causing metallic brake dust to cake on it and eventually block its retraction. Our many steep hills, my tendency to favor the rear brake, and my frequent riding on sandy hard pack probably aren't helping.
He's working up some options for a middle-tier brake upgrade without steel pistons. The SL 1's combo of flat bars and flat rear caliper mount limits the possibilities.