Okay, this is kind of a work in progress as to what I think you are trying to do. If I am wrong, let me know.
Looking this over, there are a couple things I could see being modified:
1. You might be able to save some money by not installing MOD-RA. If MOD-SA and MOD-BYPASS are both closed, the return would try to pull a little air, but if there is none going into the room, none will be going out. Just to save a little money.
2. The return fan might not be necessary. If you can get an appropriate fan with the correct CFM that takes care of all required static pressure drop (grilles, dampers, supply deadvents, return deadvents). You dont necessarily NEED a return fan in the return duct if the supply fan can take care of the pressure drop you are adding. The existing furnace should already be sized for the existing ductwork - you just cant rely on it being oversized enough for the pressure drop you are adding.
3. You could remove the chunk of ductwork that contains MOD-SA. This would give you a system that does not supply 57 degree air into the theater room, but instead provides about 70 degree temperature in the room (whatever the adjacent space is at). You would then be able to remove all dampers and just run the fan(s) off a tstat located in the theater room. This option is cheaper, but at the expense of never having nice cold air enter the theater room.
After thinking about this a little more, if you do end up connecting to both the supply and return mains, you could still size your fans at the higher airflow. It would just cool the room down faster than it normally would - you may rob a little cooling from the rest of the zone when the fans are on, but if they are running that probably means people are in the theater.
If I am way off base, let me know and I can just delete this post to avoid confusion. Sometimes its easier just to make a quick sketch to think things through.