Bitcoin requires a linear history (apart from the genesis block and the currently latest block, every block has exactly one parent and one child). If a block has multiple children, the one with the longest chain (of children) is selected in turn. The child (and its subchildren in turn) that is not selected is called "stale", and its transactions are effectively undone.

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.