Here's my theory on all of this:
1. Sharon was very intelligent and seemed more mature than the average girl of her age. She graduated in 1986, most people are 18 when they get their diploma putting her year of birth in 1968. Lori Peugot was 2 in 1969, making her a 1967 birth, or one year older. So could Sharon really have been 19? And more importantly, when did she celebrate her birthday? Did FDF ever even celebrate a birthday with her?
2. If Hoss did indeed let Linda and Lori/Sharon go, we don't know where he did it. It could have been anywhere, and I doubt that people just left off on the street would be able to be recognized or whatnot. Hoss would have probably taken ID. I recall FDF had some places he claimed to have gotten Sharon (I believe Chicago was one of them off the top of my head, I don't have Matt's book sitting next to me right now so I can't check), but with a woman who was out of a job possibly she would do anything to get money and a place to live. Perhaps she did go to that area where FDF mentions snuff films. I also am wondering about Chicago because Hoss was captured in Iowa, and if he went from the mid-Atlantic region he may have passed through Chicago or at least Illinois.
3. I see a resemblance to Sharon in the picture. What we should do is have one of those age progression websites progress Lori to 8 years (that 1975 photo would put here there), 17 years (the photo on the cover of Matt's book and the default on the new Sharon Marshall Myspace), and 19 years (the 1986 grad photo that is in Matt's book).
4. This theory does leave more mystery years for us: 1969-1973. What happened to Linda and Lori? Sort of like the FDF years from 1973-1975, although I am guessing that FDF would have abducted Lori/Sharon in 1973 because she would have been only 6 then. A 6 year old is easier to "brainwash" than a 7 year old mostly because children can take big leaps cognitively in that year of life--they enter a new stage of Piaget's development, and as much as Piaget has been criticized for his lack of continuity in development, I do agree with the guy that intillectual abilities flourish between 6 and 7 based off personal experiences.
More food for thought: I went to high school with a girl who seemed very mature and was very advanced academically. From what I've read about Sharon her personality seemed very similar. Eventually I found out she was a few months older than most people in her grade. This made me wonder if her intelligence/maturity was just natural or if it was due to being older. I'm thinking the same thing for Sharon here.