I had a visit from a Daikin engineer today.
He installed new firmware on the outside unit that is supposed to help the response time of the expansion valve.
I also reported my concerns about the inlet water tape heater that I mentioned here:
The engineer has fixed the wiring for that. It sounds like some jumper or connection was set incorrectly.
I disconnected ESPAltherma before the engineer visit, so as not to freak him out. 
I’ll reinstall it over the weekend and see if I get closer correlation between R4T and the other sensors. I have a suspicion that the tape heater on the outlet of heat exchanger might also be enabled…