I've always assumed the stepfather was responsible for Kellie's disappearance, due to his strange behavior and treatment of his stepdaughters.
If he's not involved, I wonder what Leslie Allen Williams was doing and where he was at the time Kellie went missing.
Williams is a serial killer who at different times lived about 2 miles from Kellie's high school and in apartments across the street from her high school. The Twelve Oaks Mall, where Kellie was last seen, would be the main mall that served that area, and still does, so he has probably been there.
Michigan's offender tracking is called OTIS. I don't think a direct link to his page will work but perhaps the main link will. If not you can google Michigan Offender Search OTIS to get to the site and enter his name there to see his rap sheet:
http://mdocweb.state.mi.us/otis2/otis2.html
It looks like Williams' adult offences (don't know about a juvenile record) begin in 1971, when he was about 18 years old, for B&E. He served about 4 years and has been in and out of prison up until he was caught for several murders in the 90s. His offences escalate through the years: B&E, felonious assault with a dangerous weapon, 1st degree criminal sexual conduct (in the mid 1970s when he was 22), up until the multiple kidnapping, rapes, and murders in 1991/1992.
He was released in September of 1980, reoffended, and was given another custodial sentence in September of 1983. Kellie went missing May 20, 1982, right in the middle of the time that this convicted rapist he was free, and possibly back in the area.
I do know that at one point he lived with his grandparents in Commerce Township (at a location that had a Walled Lake mailing address) the place that was a couple miles from Kellie's school. Sometime near the 90s murders, he lived at an apartment complex in Wixom across the street from Kellie's school. In 1992 he lived in Redford, which was 20 miles from his usual location.
I don't know exactly where Kellie lived when she went missing, other than the general locations of West Bloomfield and her boyfriend's home in Walled Lake. But I kind of thought it was worth looking into whether Williams was involved, since she went missing during one of the times he was free, and he was a life long habitual offender who escalated to murder.
What I remember of Williams' crimes was that in the Central Park in Milford, he tied up a 15 year old girl's boyfriend and abducted her. He took her back to his apartment in Redford where he raped and stabbed her. He drove her to a field in Milford and buried her. He raped an 18 year old acquaintance of his in the South Lyon area, and when she threatened to tell, he killed her and buried her in Milford. He went up to Gennessee County and raped and killed two sisters, about ages 14 and 16. He buried them in a cemetery there. Their bodies were later covered even further, by cemetery workers. Williams had to take police to the area to find them because of that.
There is a very large cemetery right across the street from Twelve Oaks Mall.