I could pull the valve, use the shutoff above it for the cold, and see what comes out? I'll have to think of a way to contain the water... but that's easier than redoing all the extra pipework.You eliminate 2 check valves and extra piping thats a good thing ! but he had a working system I dont see how that caused water to be at a 99 degree max . 150 degree water at the valve and 99 coming out makes me wonder if there is almost no flow at the valve . Assuming a good valve and equall pressure the water has to mix but if the hot line is plugged near the mixing valve maybe it would not reach the 120 setting or what ever you set it at also asuming its cranked all the way up
There appear to be cleanouts at the y check valves. I could open those all up and make sure nothing is stuck as well.