Yesterday the engine went into high idle mode again, but pin 30 measured something like -30 mV to -40 mV w.r.t. local chassis ground. So, that's not it either.

Yes, disconnecting and reconnecting the ECU always seems to have a beneficial, though, temporary effect. I just remembered that about 6-7 months ago, I had an issue with starting. The engine would start fine but right after the fact would not rev up but die. That happened 2 out of 3 times, the other 1 out of 3 attemps would lead to a normal engagement of the IAC picking up the revs and settling it to 750.
The other 2 out of 3 times, I had to give quite some throttle to get a succesful start, but then the first few seconds would lead to some spluttering. After that had cleared, the engine would run fine until the next start. Temperature had no effect, it would happen cold or warm.

When I first started to look into it, I had disconnected the ECU. Could not find anything wrong. After plugging it back in the problem had cured, and hasn't returned since.

I am now quite leaning towards the engine load signal. It's one of the bent pins (on the ECU side), but it's also digital. How robust would this signal be in a less than ideal contact?