Heat pump is not compatible with a water-bearing fireplace

The single parent will really be happy about that.
It's worth going to the social welfare office.

Hello again!
Repeated questions / difficulties
If there is no user for the hot water, meaning no one is washing hands, showering, etc., how can it be that the hot water drops from 54 to 46 degrees within 1 hour and the heat pump switches on?
The last few nights have been really good thanks to the new setting. Yesterday I stoked the stove from about 4 to 9 pm. Our living room was then super warm at 25 degrees, in the buffer you didn't notice anything again, the hot water did not get warmer but on the contrary, the heat pump switched on about every hour during that time to make hot water. Something just doesn't add up here, does it?
The stove should support us and save electricity. Currently, we are burning wood and need more electricity when it is burning...

If there is no consumer for the hot water, meaning no one is washing hands, showering, etc., how can it be that the hot water then drops from 54 to 46 degrees within 1 hour and the heat pump turns on?

Sounds like a continuously running circulation.

The last few nights have been really good thanks to the new setting. Yesterday, I stoked the stove from about 4 to 9 p.m. Our living room was then super warm at 25 degrees, in the buffer you didn't notice anything again, the hot water did not get warmer but on the contrary, the heat pump switched on about every hour during that time to make hot water. Something just doesn't add up here, does it?

That shouldn't be the case. The flow and return connections and the temperature sensors were changed again, maybe something went wrong? If there is no permanent hot water circulation, the temperature shouldn't drop. Unless the tank is only partially heated and has, for example, 55 at the top and 35 at the bottom, then of course you have an equalization over time. The heat from the stove doesn't reach the buffer at all? Is heat being decoupled at the 3-way valve, is the flow from the stove to the buffer hot? The stove feeds in at the top, that should actually be noticeable immediately in the hot water. Because of the direct supply of the heating circuit via the heat pump, there can be delays or problems. It can then take a while until the buffer is sufficiently charged so that after the hot water the heating also gets the heat from the fireplace. I myself am not a heating installer or building services engineer. If it is not due to circulation, few people in THIS forum will be able to help you further. Then I would recommend the home technology forum, that is "day-to-day business" there.

Should I just let my food spoil? Sorry, but we don't have a cellar. Feel free to tell me how I can do it better / correctly.

If you don't have a cool and slightly humid cellar like in an old building, then store the food, for example, in an earth pit or in the cold garage, the drafty garden shed, etc. As far as I know, you cannot do food storage in a new building. The conditions simply aren't right. You must of course make sure to store it so that no animals can get to it. But that is doable and has been working for centuries after all.

I wanted to give you all an update. After we finally found an external heating engineer, he was able to determine relatively quickly that something was seriously wrong and that the power consumption was far too high. Solution: our heating engineer connected the buffer tank according to the Viessmann scheme. The only problem was that it is not a Viessmann buffer. And lo and behold, when using the instructions from the buffer manufacturer Alpha Innotec, everything works exactly as we had imagined. Power consumption reduced by more than half, and when the Swedish stove is on, the heat pump also no longer runs. So thanks to everyone who didn’t think we were crazy but looked for solutions with us!