Heating reliability during power outage - possibilities?

I think everyone will have different worries

You just have to prepare comprehensively. Water and food for 2 months, also medication. You can easily store everything in any shed.

- the hybrid SHT runs without power as a conventional stove in case of a failure

The chimney sweep said the dual occupancy would be a problem because of the recirculation system, so that's out of the question

An additional chimney with an additional stove would be an option but not a good one because it would never be used and would rot.

The pellet stove I have now heats very well, but it needs power...

the hybrid SHT runs without power as a conventional stove during a failure

And by what mechanism is the produced heat delivered to the living spaces?

No, it does not run without power. It conducts water. What happens to water when it gets very hot? It boils/evaporates. To prevent this, there is a sensor that flushes the oven with cold water. The water goes into the drain.

Without a pump/power, the oven must not be operated.

I think you believe that only the pellet feed requires power. That is not the case.