Why are you making RED attack and BLU defend in payload? It's generally a good idea to stay with the way the game has always been: BLU attacks and RED defends. This goes for all maps where there's an attack/defend scenario (so Attack/Defend and Payload). If there isn't a logical reason to have RED attack in a scenario where BLU normally does that stay away from it and let BLU do the attacking.
If you do have a reason why RED has to attack, you have to make sure that all entities that normally make BLU push and RED defend are basically inverted to a point where RED attacks and BLU defends, so that means switching teams on the trigger_capture_area attached to the bomb, set the team_train_watcher to RED and update the team_control_point_round so that it restricts BLU from winning, as said above. And of course, change the BLU bomb model to RED's. But again, if it's regular payload stay with BLU attacking and RED defending. It saves you from having potential headaches with entities later on because you switched all the payload-specific entities to RED and it doesn't confuse players playing on your map.