The Woman in White: unreliable dim-witted narrators

Earlier I wrote about Marian's stupidity. 
Wilkie Collins needs other tools for his novel. 
Consider all the descriptions: I believe what happens in real life is that tiny details are either unnoticed/ dismissed as insignificant, forgotten and left out of the narrative, or noticed and remembered, seen as suspicious and then pondered over. That a character sees something, then pays it no thought, and yet still remembers it later and writes it down but doesn't treat it as meaningful, is odd. I'm thinking of Marian realising that the letter can be opened right away, Marian noticing the inexplicably strange spot of the seal, Marian having a vague feeling that she's being followed, Fanny noticing the letters are strangely crumpled, etc. There must be more examples that I forgot about (not regarding them as important, perhaps?). The characters thus have to be observant enough, with a memory that is good enough, not to overlook or forget these trifles- Wilkie Collins needs everything to be there, but they also have to be dim-witted enough not to put 2 and 2 together and not to realise their significance, at the time, to know what to do. So far, among the narrators, Hartright's probably the quickest. Other witnesses like Marian, Mr Fairlie, Fanny, Mrs Michelson (the housekeeper) are amazingly, intolerably slow that it makes one impatient after a while. This slowness is necessary for the plot and the presentation of all the important details, it's just repetitive and sometimes, how should I put it, a bit false. 
Hopefully next time I'll write something positive.