Some thoughts:
1. Asha's dad was up until 2:30 a.m. the night she disappeared. What if Asha had arrangements to meet someone earlier, but was unable to leave because her father was up so late? I wonder what a normal bedtime is for the parents? IIRC, the AC went out that night, so the circumstances were unusual.
What if Asha went to meet up with someone as soon as she was able to sneak out of the house, but the other person had left by then because he/she decided Asha wasn't going to show up. Asha may have continued walking around, trying to find the person she was to meet up with. Being cold and wet, when she saw some buildings in the distance, she may have taken shelter in the shed. What other buildings were between Asha's home and the area of the sightings and shed in 2000?
If the person Asha was to meet up with was no longer at the agreed-upon location, maybe she was spotted and abducted by an opportunistic abductor sometime after the reported sightings. But, if the person Asha went to meet wasn't her abductor, I would think the person who had made plans with Asha that night would come forward since he/she is an adult now... unless that person now fears getting into legal trouble because Asha was a minor and he/she was older and may not have had completely innocuous intentions.
2. Maybe the T-shirt and book were found in an abandoned house/building outside of Cleveland County, but the library book led to an association with Cleveland County. Since investigators would be able to trace the previous owner of a house, maybe the residence or building was used by squatters. Or, Asha's abductor may have used several aliases. Since Asha, if still alive, would have outgrown the book and T-shirt when her abductor moved to a new location, they may have left the items behind. The items could have been found when the house/building was slated to be demolished, was renovated, or new residents moved in.