I understand why you are focused on fueling, but don't discount coil sticks/plugs.
My Gen2 always behaved, but my first Gen1 had a similar issue to yours. The filter baskets on each injector were partially blocked with mainly copper debris, presumably from the fuel pump itself. A fiddly DIY job - I just cleaned them up, but I think there are kits available now that allow you to replace the filter baskets & injector seals.
What is the story with the trader you purchased this from? Was it theirs to sell, or a commission sale. I sense they are avoiding responsibility & kicking the ball down the road. hopefully I've got it wrong.