Hello Karspars,
I find your project very interesting and found it whilst Googling with much the same thought in mind. The idea to achieve good results by tweaking and misusing somewhat cheaper existing stuff rather than buying expensive and over-the-top off-the-shelf always appealed to me.
The current gas-crises in the EU already triggered me to buy two 3,5 KW MHI single-split units for room heating.

For the warm water supply, your setup could be nice, no doubt here. I’m thinking to keep using of my gas heater for sanitary water only, until I find a good alternative (much like yours).
The controlling/software side would be the hardest for me to master, and I can imagine that that is the trickiest part. As I understand it, you just incorporate a plate heat exchanger in the evaportor feed line, and leave the usual inside-unit in place and operational. Then, the inside-unit would function as normal unit as long as there is no heat-exchange (no condensation) at the plate heat exchanger?
The plate heat exchanger would then just be an inactive part of the feed line inner volume (maybe asking some extra refrigirant to make up for that extra volume)?
As soon as colder water would flow through the plate-exchanger, it would start to exchange heat and the refrigirant will condensate. I can imagine that just a slight waterflow, and thus only slight heat exchange, would still enable the inside-unit to partly function and heat up air. However, I can imagine that the unit’s electronics/program would go “haywire” without your tweaking, and that programming can be quite a challenge.

Having an inverter-driven heat pump is certainly nice, but what I’m also asking myself is, whether or not the simplest outdoor unit could do much the same, be easier on the programming, and still be energy efficient, when used as below?

I ask myself, whether or not an outdoor unit has a certain “speed” at which it will always be most efficient (highest COP), regardless of the outside temperature and without any adjusting to a changing heat-need. If so, and now that water is involved (no heat surplus), then it might be best to try using it much like a “wood fired stove”? Just let it run as economically as possible and just live with the changes in heat-output.

I have a gas-fired central heating system. Suppose that I use only an outdoor unit and connect two plate-exchangers in series, one for central heating system water and one for sanitary water (like you do). Then, I would be able to direct the heat output towards either one by simply adjusting the secondary waterflows (I imagine). A small outdoor split-unit wouldn’t heat up the central heating system water much, but “what’s in the house is in the house”. And for sanitary water a small outdoor unit would work out just fine like in your setup. Then, it would be possible (am I right?) to just put an outdoor unit on a fixed slow economic pace and give sanitary water first priority, with all surplus heat flowing towards the central heating system. In essence. the central heating secondary pump would be on all the time, except when the sanitary water pump comes on. The outdoor unit’s “speed” could be manually set and changed if needed. It wouldn’t automatically adjust for changes in heat-need but would be more like a wood-fired stove, just heating in a steady pace. Would this make sense?

Another thing that I’m not sure of, is what partial or complete condensation in the plate-exchanger would do to the existing indoor unit. It would then receive liquid refrigerant like the outdoor unit is used to, and might not be able to cope with that? And existing internal safeguards (temperatures, pressures) might hinder?

Best regards,
Andre