Hi....it is possible for the tonsils to reherniate. I have not heard of a herniation to be completely gone post op....typically the area is widened enough for CSF to flow. How long the tonsils are is not concerning, only if it is wide and obstructing CSF flow.
You will always have Chiari.....as Chiari is the malformation of the skull not the herniation.
May I ask why you had another MRI 2 years post op?