Could someone explain what a "stale chain" is and what the implication of this is?
This could be a used to try do a "double spend" (send X bitcoins to address A, rollback the transaction, send the same bitcoins to B instead). However, according to the OP's analysis that hasn't been attempted (yet), and the active chain is far enough ahead that it seems unlikely that the stale chain would ever catch up and overtake it.
1. Longer block times, so it is pretty rare that multiple miners are finding multiple valid consecutive blocks to compete on separate chains
2. It is against their incentives. By being apart of the losing chain, they have used thousands in hash power and missed out on possible tens of thousands that would come from the valid chain.