Given the size of your room I REALLY recommend 4 drivers!
You can certainly wire 4 DVC drivers for a correct load for a plate amp. Also know that 4 drivers will play atleast +3dB louder than 2 drivers. So no you won't need a more expensive amp, and yes you do need 4 drivers.....
Until your wife is willing to accept the concept of 'compromise' I have no additional suggestions.......sorry.
I knew I didn't explain it well enough, sorry, the back wall of the closet has the twelve mounted on a test baffle as a normal IB. Behind the wall of the closet (sheet rock/studs) is the furnace room for the backwave. With the tempests I would do a simple manifold venting into the room through the closet. I know, I need to take some pictures, the one I posted just shows the closet door, not the IB itself. I have checked out your cult site many times so I do have a basic understanding of IB's, I just need to work on expressing myself. THANKS.
Thomas, thanks for the advice. Sounds like you are recommending the Dayton DVC 15" over the Dayton IB 15" for a 4-driver IB. That's probably the way I'll go, and the more I think about it, the more I'm leaning towards a cube manifold approach. I need to run electrical wiring through the wall where the IB will be located, so having less wall height taken up by the IB is better.
Daniel, good luck with your install, and let us know how it goes!