It’s possible to heat things like return water or cold water directly and then use heat exchangers.

Temperature differentials also doesn’t matter much because temperature doesn’t directly behave like voltage, if your hot source is too hot you simply use less hot water. You’d likely end up with multiple heat pumps, but each one can be smaller as they cooperate.

Here in Sweden we already always distribute to homes at 70-100°C and then at 50-55°C internally (again, with heat exchangers to minimize energy loss). And that works just fine. We just don’t (yet) have many systems which feed back thermal energy.