Reassembly: After powering on and running for a few minutes, found a large whitish patch on the screen. Persists after powerdown.
Turned out to be condensation, though it didn't look like it - more like fine white powder rubbed in. Humidity is very high this time of the year. Fix: start airconditioner in a room, leave it running a while, take iMac in, prise open the glass again. Condensation vanishes instantly. Keep it out for a ~15 minutes running infinite loops to get iMac nicely heated up and fans running. Snap glass back.
The ZIF display cable is tricky to get back in. I would suggest that this cable be the first one to be put back during reassembly. You need maximum wiggle room and visibility for this. Push in the display power cable *after* this one.