I’ll bite on this topic. Too juicy not to 
I am impressed with the level of experimentation and monitoring that David has carried out so far. I personally don’t think enough work is carried out exploring heat pump control strategies but I know the reasons why though.
- Most people only care about heating if it’s not working, or if it’s costing them a fortune to run.
- Manufacturers’ want their ASHP box of tricks that performs as reliably as possible for as many types of installation as possible. It might not be the most efficient strategy they use, but it is often the most reliable.
With that in mind, and based on my day job as a tech support guy for a heating manufacturer, I’ll throw in my 5p worth 
(All opinions my own). Whilst this may come across as negative, it is not meant to be. I just want to remind anyone wanting to experiment, to bear some important things in mind.
Now I did speak to @TrystanLea about this last year at the Fully Charged show where OEM had a stall. He mentioned how good it would be to be able to adjust the compressor running frequency / speed based on available excess solar PV energy, or to have an influence over the defrost cycle of an ASHP.
Whilst that would be great to be able to do this as an experimenter, no manufacturer would be happy doing this from a warranty point of view. Priority 1 of the software on a heat pump is to look after the heat pump. Priority 2 is to provide heat to the customer!
Every manufacturer has a different strategy around defrost cycles - it is their “secret sauce”, whether it be a time calculation around the time it took to complete the previous defrost, or based on ambient temp, evaporator temp and suction pressure or perhaps around CoP.
One thing for sure with a reverse cycle Air to Water heat pump, is that when the heat pump needs to do a defrost, it needs 2 things. Good water flow rates & sufficient thermal energy from the heating circuit & buffer tank. A lack of flow or insufficient thermal energy during a defrost cycle can cause water freezing & crystalisation in the condenser. Freezing water expands, potentially splitting the plate heat exchanger condenser. The software built into the machine will do everything it can to prevent this from happening. That might be faulting out with an error code if needed, or switching on a flow boiler / buffer tank immersion element to assist the defrost. Just be mindful of this if you are controlling a circulation pump controlling flow through the heat pump with your own on/off durations and pump speed. If the heat pump was expecting to control that pump, but in reality you are, it could cause trouble. A split condenser = a water filled refrigerant system. Not fun is an understatement!
Now I can see David has been experimenting by switching the heat pump demand on/off and slowly ramping up the water return set temperature rather than allowing the heat pump to do this quickly. I would imagine this results in the heat pump running the compressor at a lower RPM speed for longer and having a reduced cooling effect to the evaporator, potentially prolonging times inbetween defrosts & increasing CoP. What effect toggling demand on/off signals or altering the return temp set point on the fly to the heat pump makes to the defrost algorithm I don’t know (Never worked on Mitsi). One to perhaps think about.
On the topic of compressor running speeds and run times. The compressor manufacturer will specify a minimum and maximum running speed for inverter driven compressors as well as minumum run times. This is to ensure compressor longevity. Compressors contain oil and the compressor RPM and run time makes all the difference in making sure that oil gets to where it needs to go within the compressor. Generally speaking, compressor longevity is increased by reducing the number of starts per hour they have to perform. There is also a rule in the UK that should be followed to prevent upsetting the local DNO - no more than 3 compressor starts per hour. Most heat pumps have this inbuilt. Trying to override this may shorten the compressor lifespan and upset the DNO if they catch you (unlikely but still!)
Nevertheless, my inner heating geek rather likes seeing stuff like this. Would the gains in CoP efficiency be worth the time & effort spent seeking them? I would question it, but if you’re enjoying it and it doesn’t leave you with a broken heat pump, then I will enjoy seeing the fruits of your labour.