@HmmOk
I DEFINITELY knew the permanence of death long before the age of 8-I had become terrified of it at around the age of 4 and slowly worked my way through the feelings but there are so many nuances to learning levels and maturity, and my IQ is much higher than average (not a brag, I am also incredibly stupid in many, many ways)!
I think she was brainwashed by Brockhill, scared of her, possibly thought because Brockhill was so much more mature and 'together' (in Frankie's eyes) she knew best. I also think she probably 'knew' it wasn't but didn't value her own opinions or trust her own morals and value judgments-as you've said, a result of a not-great childhood. Was Frankie even allowed to be her own person, did people think they'd done right by her by not allowing her to be? Who knows.
IQ is a measure of intelligence in some but not all ways. You can have a low IQ and be empathic or a high IQ and not understand unrelated things. I know people who might be labelled 'Thick' who have an emotional range higher than mine and other much more 'intelligent' people. As others have said, Frankie could be manipulative, secretive-she has held her own in arguments with Brockhill. As awful, awful as this case is, the nature of the humans involved does cause fascination in many of us, clearly. Frankie's IQ has become an object of fascination.