when player a takes over a region with player b's active race, does player b get his troop back or is it considered dead and be placed in the tray?
Player B will get the troops back minus one. So if there were three troops attacked, one troop is returned to the tray and two will be returned to player b. Player B can then reinforce his existing troops wherever he wants after player a's turn.
If there was only one troop on the location its gone for good.
If player A attacked player b's LAST location and had no places to reinforce with the troops returned to him he must enter the board (or decline) as if it was a first conquest when it becomes his turn again.