Nice write up on the ZG. I was there on Saturday saw the car in person as well. Besides the valid points you made on why the ZG sold for not as high as anticipated, I think the main reasons for it not selling higher was due to its condition and previous repairs done to the car.
When looking through the opening of the g-nose it was apparent that the radiator support was silver. At first I thought maybe the car was originally silver and just made into a ZG clone. But after opening the hood it was obvious from the poor welds and messy application of seam sealer that the front support had been replaced. The sub-par repair also caused fitment issues between the front headlight buckets and the fenders. Both floors had been replaced with just plain sheetmetal instead of stamped repros or floorboords from a donor car. The driver-side door jamb looked to have been previously repaired, possibly due to rust. When I opened the hatch there were loose rust flakes found all over under the carpet. An electrical fuel pump that was fitted underneath by the factory mounting points was jerry-rigged to a thin sheetmetal strip one would find at Home Depot. The body had numerous dings and wavy body panels, and the paint, while it still had a shine, had many spots where it was blemished or cracked. The hood emblem was the S130 version. These issues along with others I didn't note I think caused any potential buyers present to be turned off from buying the car.
I did try to ascertain if it was an authentic ZG or a clone. With the exception of the radiator support that was replaced by a silver one, everywhere I could inspect without ripping out interior panels led me to believe the car is an original GP Maroon car. I even looked under the liners in the tool bins and it looked to be the factory paint.
What shocked me the most was how well this car looked in the RM Auction photos vs seeing it in person. I was expecting the car to sell higher due to online bidders registered in the auction. RM Auction worked their magic in those photos.