After soldering the lowest joint back together, I added the hose barb at the top (lower left in the pic) that I've been doing without for several years. I sprayed the t-stat cover's gasket surface (& later, its hose nipple) with the copper gasket adhesive, laid the FelPro 35355 in place, and then applied Hylomar so it could be removed intact later if necessary. (I don't have much confidence in my plumbing soldering.)
UPDATE: No leaks after 5 years.
