I'm not quite sure what you meant by your question - about boilers using gas vs water. Are we talking about refrigerants being liquid or gas at a particular point in the cycle? circulating hot water for indirect water heaters? burning natural gas?
I'm actually tinkering with a similar concept - trying to use both ends of heat pump along with seasonal energy storage - (looking at insulating a tall, well-insulated tank to store cold, warm and hot water at varying proportions depending on season - making use of water's relatively decent insulation via thermaclines). I'm modifying a hot-water heat pump to replace the fan coil with a flat-plate exchanger to make it a water-water heat pump with a relatively small refrigerant circuit - and using plain water for everything else - heating, cooling, and wasting excess heat/absorbing heat as needed via an outdoor fan-coil using water. Fortunately, I'm at a frost-free location, but would think this could be adapted with sufficient storage if temperatures are often well above freezing for periods of time in winter. Quite a bit of work ahead still but happy to share more if there is interest (perhaps a different forum).
I don't think there is much to gain from adding a thermal plate to a PV panel - depending on the local climate and working fluid you would be dealing with a lot of connections and extra refrigerant (a hazardous material under pressure) + complexity and extra controls. But ignoring all that - in the circuit you provided, if you're using both the hot and cold ends of the heat-pump, it would add a bit more superheat on the compressor input which should raise its efficiency a bit to reach a high outlet temp. Running the panels a bit cooler also increases their efficiency. However, if you're mostly using AC with little hot water, you want the condensor as cold as possible for best efficiency - so you'd want to disable this much of the time.
Alternatively your PV-thermal-plates could run an additional loop of water/antifreeze - perhaps connected to a coil at the bottom of that tank to preheat incoming cold water. I would design that tank to directly circulate water via inlets/outlets at different levels, instead of being static with many different coils. You'd need a coil at the top for potable hot water - and also consider how to deal with any parts of the system that might freeze.
Note that your "condensor" is in the wrong place. It isn't a discrete piece of hardware, just the hot heat-exchanger on the output of the compressor (the outdoor fan-coil in a typical AC unit). The expansion valve keeps this side at high pressure, allowing the refrigerant to liquefy (condense) at a high temperature as it heats its environment. For example, if this heats your hot water, then as the water heats up, the refrigerant will end up being pumped to higher and higher pressures to reach a higher temperature, generally at lower efficiency and higher system stress. The system will have enough overall refrigerant to operate of the range of temperatures it is expected to manage.
Have a look at a heat pump cycle on a refrigerant enthalpy pressure chart to get a better idea at what happens during the 4 portions of the cycle.