If an unstressed Tie Reaper(or Striker) executes its compulsory bank or straight 1 manoeuver and ends this manoeuver off the board, does it count as destroyed/fled or can it continue to reveal its dial and complete its chosen manoeuver(possibly surviving)?

Rules Reference 1.0.4 page 11:

“A ship flees if any part of its base is outside the play area after it executes a maneuver. A ship that flees is removed from the game.”

And:

“Before a fleeing ship is removed from the game, the only effects it resolves before being removed from the game are effects that trigger when it flees.”

So yes, the Reaper / Striker is “removed from the game” when the compulsory maneuver takes it off the board, causing it to Flee.