Can followers be used to keep stolen items from "the law"?
Solution 1:
NPCs cannot go to jail unless it is part of a specific quest. If they participate in the crime (assault, for example), they will be approached by the guards. They then either have to pay the fine (if they have the money) or resist arrest, which will probably get them killed. Either way, stolen items in their inventory will also disappear.
However, if they have not participated in the crime (ie; your pickpocketing), then the stolen items are safe.