While you have a few good points, the major flaw is that is NOT a pewter. Honestly, its more likely that its a normal pastel then a pewter. Pewters have a completely different look to them and I would bet a decent amount of money there isnt any cinny in that snake.
As to it being highly unlikely there was a fluke in her record keeping, its not terribly uncommon. Mistakes happen and there are usually multiple clutches I read about each year (including a few this year already) where that exact thing happens.
Finally, how does it not look like a super? It has the right color and the right blushing, but not a ton of it. Supers have a wide range of blushing, just like pastels. You see some with a ton of blushing, and some with little to none.
Bottom line, I would say that you most likely have a super pastel, slight possibility of having a VERY nice regular pastel and an almost non existent chance at a pewter.