First things first . . . don't spend money on this yet.
It sounds like you found your blower motor resistor package mounted to the firewall in your engine bay just right of center. The harness that attaches to that point comes directly from the blower switch. You also said this:
I have pulled the blower and the attached pre-resistors and everything tests OK, all four speeds.
As you and
correctly stated, the problem lies with the switch or the wiring from the switch to the resistor pack with one tiny little detail left open to speculation on the resistor package, the overheat circuit. Give me a little time to review that.
If you decide to dive into that HVAC panel, start by removing the ashtray. Next, remove the 2 screws that hold the ashtray frame in place. The frame is removed by pulling on the right side, allowing it to deform a little. There is a 2 pole plug on the left side that must be removed - don't panic if that plug vanishes, it is easy to get to later. I have not worked on the ROTW (rest of the world) version of the HVAC system that
outlined in his post. It seems like you have this version. There may be two small machine screws in the ceiling of the ashtray cavity that anchor a clip for one or both or those knobs on the panel. I don't know if ROTW models have this - they are on the temperature/vacuum switch (AC switch) on my car.
Pulling that HVAC panel is a delicate operation because the trim around your slider handles is very prone to breaking - even new ones (which probably have been sitting on a shelf for years and are no longer new). Those have to come out to reveal screws that hold the panel in place. I use picks to unscrew the metal nut behind the knobs.
Once that panel is off, a continuity check will reveal a wiring integrity problem. The same can be done on the pins of the switch.