B&W should be able to sell you a single 685, so just email B&W directly and they can help you out - like talking to your dealer, etc.
I would get a 685 as a center if I could buy a single, not if I had to buy a pair. But I bet B&W customer service can help you out in some way.
If you compare the 684 vs 683 again for an hour and still feel the 684 sounds clearer or better, than buy the 684. Trust your ears, as long as you have plenty of audition time and materials.
do you know if i can lay the 685 on the side because i wouldnt know where to put it if i have to stand it up right?