I deal with adults with learning disabilities among other problems and I consider Misty pretty much an adult in that respect. She's seventeen, nearly eighteen now and while teens of that age may still be maturing emotionally, their brain development and cognitive functions are as good as they're going to get, or very nearly so. They may educate themselves and learn a lot after that age, and usually do, but in terms of physiology or neurology their brain function isn't going to improve much.
Learning disabilities include a variety of complaints and difficulties and it would be preferable to have a more detailed description of the particular difficulties involved.
From the examples of her writing that we've seen it seems like she could have dyslexia, difficulty writing and possibly reading as well, but that alone would not cause her verbal replies to be inconsistent or in contradiction with the evidence. She also may have some problem with spatial relations (she struggled with inches, feet, whatever...) which might make it difficult for her to explain how far exactly everybody was sleeping, but IMO it doesn't explain the other changes in the story.
Memory difficulties would obviously compromise the reliability of a person's testimony but in my very subjective opinion the relatively fluent way she produces her story doesn't resemble the verbal behavior of my clients with severe memory complaints. IMO she acts like she remembers and never complains that she doesn't remember. If she has memory difficulty that explains her inconsistencies she seems to have problems with perceiving her own difficulty as well because she hardly ever refers to any difficulty remembering. (Sometimes she says "I don't know" and leaves things blank but my impression is that it's intentional. Dunno, the interview material we have hasn't always asked the right questions to clarify this issue.)
Anyway, if she has memory difficulty I think it could make her a bit vague about times and details,things like whether she washed blankets for 30 minutes or 60 minutes, whether Granny came at 7 or 7.30.pm., but it's more unlikely that she'd have been misremembering things like whether she was home last night or not, when she was questioned about it the next day or the same night. You don't need a memory difficulty to be vague about exact times though, it's enough if you just don't pay any attention to the time when doing those things.
The 911 call had the huh factor her unable to answer many questions about Haleigh. Not remembering her date of birth or her height and weight could be an example of a memory difficulty, but then again she might never have known these things in the first place, or she might have been under the influence of something that altered her capacity to reply.
Memory-challenged people who suffer from confabulation may be very fluent when make up stories to fill the blanks in their memory but IMO she doesn't have that, because the essence of her story has stayed the same over eight months and a confabulating person wouldn't be likely to be as consistent, they'd make up a new story each time. Also their stories may contain elements that are clearly fantastical, clearly figments of their imagination or associations they catch somewhere. They tend to make it quite obvious to an observer that it doesn't make any sense.
Language difficulties might present as difficulty in understanding the questions or difficult words, and I think we have some evidence about that ("numerical, what's that?") but IMO she has understood most if not all the questions that she's been asked in her interviews pretty well because most of her answers have been to the point and she has seemingly answered the question that she was asked. Comprehension difficulties often become evident because you ask a question and the person replies but she appears to reply some other question because her answer doesn't relate to your question. Difficulties with vocabulary, grammar skills, complex thought, verbal expression or general intellectual level might make it difficult for her to relate her story in a precise manner or in exact sequence and cause ambiguities or make a person use vague wording that changes between tellings although she really means the same thing. I think she may have some problems with some of these things (just an impression having observed her talking) which may introduce some ambiguity along the way. But we have also observed her in some interviews to be quite fluent and confident, so it's not very bad, IMO.
IMO language difficulties should present no unbridgeable obstacles to Law Enforcement because the majority of their clientele are not Cambridge scholars and they should be used to dealing with people with less than perfect speaking and comprehension skills since they do it every day. If it was just a matter of getting her to understand what she's asked or understanding what she says they should have been able to adjust their questioning to achieve that.
If it was just a matter of being sketchy, inconsistent or vague I might write it off as a possible consequence of a learning disorder but they said they have physical evidence that she cannot have been doing the things she said she was doing. The things she was asked to discuss weren't nuclear physics, she was asked to describe what she did the night Haleigh went missing. Cleaning, cooking, watching kids play, Granny stopping by, watching movies, washing blankets, sleeping... This sort of thing is well within her linguistic capability, IMO, and if she got that wrong it's not because she can't understand what was asked or couldn't say it right.
Also, in my opinion, "sly" and "slow" are not mutually exclusive. People with learning disabilities or mild intellectual disability are usually quite able to lie, even if they can't spell "prevaricate". Sometimes on their own initiative, sometimes prompted by others.
