http://www.xfire.com/video/49a759/
This is pretty much what happens throughout your "complaint". As you can see, you can swap after shooting as long as you hold down the trigger for x amount of time. In heavy combat, the timer sometimes gets bugged, and starts counting from an earlier point in time. This is not an exploit, just a bug that happens sometimes.
As for Sergio, the interior markers are buggy as hell, and often require you to move away from them to make them work - this is seen when Sergio runs to the vending machines and back, and is able to exit.