Desuperheater vs Heat Pump Water Heater
I've been looking into a heat pump replacement.
You said "However, when the heat pump is in heating mode, the exiting refrigerant line is actually colder, not hotter, than when it enters. As a result, you cannot heat water during the winter."

That will depend on the location of the internal connections.  The HP models I have seen with a desuperheater ( hot water generator ) place the HWG after the compressor before the reversing valve.  That means the pump always runs forward, the HWG always has hot refrigerant to use and it works in summer and in winter.  The reversing valve does just that - in summer it sends the leftover heat to the outside and extracts heat (cools) the inside.  In winter it sends the heat to the inside and extracts heat from the outside.