In the ideal case, it would then feed directly into the underfloor heating and not into the buffer tank, in order to at least ensure a certain base temperature.
I have read this several times now and don’t understand it. We have exactly that: an MFS, which is heated by stove, solar, and heat pump. It works. Yes, the control system required quite a bit of experience from us.
The heat pump alone on the buffer is pointless; then it heats the buffer. But once the buffer is warm, the heat pump can maintain the temperature.... whether it compensates a delta-T in the buffer or in the underfloor heating should not matter to it.
The problem when you connect the heat pump directly to the underfloor heating is controlling the flow temperature. The heat pump is the only “intelligent” device in the system that can calculate and control a flow temperature. If you separate it from the buffer, then you might overload the buffer with heat (via stove or solar), and who then regulates the output to the underfloor heating?