After replacing a recirculating pump (and pressure regulator), shower won't run hot
Here's the answer:  The problem was solved when:
1)  The recirculating pump was connected the right way round to suck water back to the water heater from the far side of the house AND...
2)  The new digital timer was programmed correctly and left on AUTO not OFF....! 

Still can't figure out why it got worse simply by replacing a non-functioning pump with a good one, that was turned off (taking 2 minutes instead of 15-30 secs).  But as long as it's better now I don't really care!