One thing you're overlooking here is the connection between the hot end and the control board. Perhaps a ribbon cable has gone bad and one of the wires inside is on the verge of snapping completely in two? This would cause inaccurate, high resistance values giving you a lower nozzle temperature as well as give you intermittent and worsening issues. You could just swap it out or use a multimeter to check the resistance from one end of the ribbon cable to the other.
Frankly I'm impressed you've managed to troubleshoot this far. Just normal/common printer issues often cause me to step away from printing for months at a time due to the frustration.