Alrighty. Unplugging the solenoid had no change in driving behaviour. Idling is incredibly rough. I tried unplugging various sensors but saw no impact.
I got in contact with Henk from Barchettaparts, as he can do ECU testing for no charge. I've also found a supplier with a replacement ECU that has the same part number as mine.
I've sent the ECU to Henk. The issue may not lie with the ECU, but seeing as its a free service, it makes sense to strike it out. If it comes back as having no issues, I'll pop the car over to my parents and work with my dad to falsify signals coming from the IACV, MAF, TPS and temp sensor. This will help check whether the ECU is receiving dud signals from these sensors, either indicating they are bad or that the wiring is bad. If that shows nothing, then I can only think its a vacuum leak at the base of the inlet manifold
If the ECU does come back as faulty, then I can grab a replacement.