I am trying to ascertain the timeline of the stealing and recovery of Syamantaka and whether it happened before or after the Mahabharata war. Is this mentioned in any of the scriptures ?
Syamantaka mani episode happened before Mahabharata war.
In Mahabharata vana parva, when Pandavas are in the forest, Krishna visits them along with his wife Satyabhama. So by this time, the syamantaka episode must have occurred.
Hence the conclusion.