Also, the passage from TGO says "Three fingers had been lopped from his right hand." Lopped, not merely missing. I think there was also mention of scars (supporting the lopped off description). If it was congenital, there wouldn't be scars.
Anyway, in one of her sections in TJE (I think), Esmi reminisces she could tell relatively early how 'human' her children were (or weren't). Pretty sure a Dunyain assessor, whose job it is to determine such things, could tell pretty much instantly if one of their kids was defective or not.
IMO