I have created an open source heat bed thermal emulator/simulator that is reasonably accurate. It illustrates many of the issues that are described in this topic. It takes conduction, convection and radiation into account similar to your calculations above. Maybe it can be of use to you for making approximations.
https://monkeypangit.github.io/thermalemulator/
It runs in your browser without any need to install anything.
The source code can be found on github:
My own conclusions from building this simulator and from testing is that support for a dual thermistor setup in klipper with a MIN(PID(A), PID(B)) controller is probably the simplest solution. But I have not had time to pursue this yet.