As noted, the screws screw directly to the plastic shell and hold quite well (surprisingly) this is the same method used by Scuff. the washers provide spacing and the flexibility needed when pressing the paddles.
Placement of the tacts is critical because, if they are too low on the controller, the MOBO wont sit down into the shell enough to line up the mic input port. If the tacts are too high, they will interfere with the clips on the battery cover when they are inserted.
If all that is needed to be able to use the chat adapter is clearance between the paddle and the adapter, how about using a taller tact and and additional rubber washer to gain the clearance needed. you may also need longer screws.... but hey, happy modding.
Edit: and if that works, you can change the paddle screw/washer configuration then thicken just the portion of the paddle that contacts the tact to make up the difference. this way it is all done externally which should not void the warranty on the controller.