Great how-to, Gatos - as always.
When my heater resistor went a couple of years ago, the whole ECC went haywire, randomly blowing hot, blowing cold, the blower switching on and off, air con jamming on, jamming off, stopping responding to the controls, the works. Rather than buy a complete new unit from Volvo, I bought just the transistor (the 'flying saucer' bolted to the heatsink) from Maplins, and replaced the original with the Maplin part. All of this was following a step-by-step guide that I found here on the Forum (I don't have the link handy, but I'm sure it's still available).
I notice in your introduction that the symptom you describe is solely the heater blowing air at full blast. Mine has just developed a new trick: sometimes it does exactly this. Usually, if I park up, when I start up again, everything works correctly again.
Would you say it's likely to be the resistor again?