I haven't read all of this thread, so I may have missed this detail, but could your fan speed issue be related to the HVAC switch itself on the dash?

I'm mentioning this because I had that very problem, where the switch was messed up and couldn't control the fan speed correctly. The fix was to buy a used unit on Ebay and slide it into the dash. The old switch felt fine, but it was hit or miss how it controlled the fan speed.