I know this may be O/T, but it may also be very relevant.
Sounds to me like maybe Shannon's mom (and possibly) had Williams' Syndrome. It is a genetic disorder that greatly affects the physical and mental development of those affected. My cousin woho recently died was born with Williams' Syndrome. When she was born, she was 3lbs- to get clothes that would fit her, my Moma made them using my dads thumb length to measure the waiste, his pinky to measure out the arms, and his pinter finger to measure out the legs. She (my cousin) had a normally shaped and sized head, but had very small features otherwise. At a year old, she was barely 13lbs. She was tested for varying disorders, including Downs Syndrome, FAS, Dwarfism, and several metabolic disorders before she was peopherly diagnosed, and she was alomst 18mths old before a proper diagnosis was given. This condition is almost 100% passed down to childern of adult sufferers, which could also account for issues with Shannon. My cousin, functioned at the mental level of a 8 to 10 year old, and was in special education classes. She was not by any means slow or stupid to say the least- she she just had different areas of strengths.
All I am saying is that there is any number of reasons that Shannon could have been small for her age, and giving some of the descriptions of her mother, it is highly probable there was a medical condition involved with her size/devealpment. This too could have played into what happen to Shannon, God forbid someone close to her had something to do with her dissappearence.