Hot water temp in shower/tub fluctuates
Your plumber is right... or his test was successful in identifying the problem. Since the hot water is installed in a loop system, and cutting off the return seems to solve the problem (or at least reduced the issue), then obviously there is a bad cartrige or crossed line somewhere. 

Cold water usually has a slightly higher pressure than the hot water because it has a shorter path to travel (it doesn't travel through a boiler). So, with a bad cartrige (maybe missing an o-ring or something) or a crossed pipe, the cold water would flow into the hot (instead of hot into cold). 

Regarding the test, since the hot water is installed in a loop system, the pressure is actually even lower than a non-return type system. The lower pressure of the hot water loop system allows more cold water to enter the hot water line. When he shut off the return (no loop), the problem was lessened to a noticeable extent. This strongly suggest a crossed pipe or faulty cartrige.