Final update:
I checked the schematic, there is no mux between the eDP LCD and the i5 SOC on this model.
I downloaded and read the SOC datasheet, thinking perhaps there was an external voltage regulation issue on wakeup - but the video core voltage regulation is in the SOC, not external.
I bit the bullet, and bought a replacement logic board on eBay (got one with 16GB) and just finished installing and testing it.
WORKS FINE
Therefore I must conclude the issue was within the SOC, something to do with changing power states.
I can use the defective board with an external monitor, or perhaps I'll replace the SOC in the future.
I appreciate all the helpful suggestions I received!