If you're able to, simply create a bootable USB volume (drive or even memory stick will do) containing Snow Leopard and Airport Utility 5.6. Boot into the Snow Leopard USB (restart and hold down option key), launch Airport Utility 5.6, and configure the Express. When you've done configuring, simply specify the original volume containing Yosemite as the startup disk (under System Preferences), restart, and boot back into the Yosemite volume. I did it this evening and it works fine.
@TRA is it the setup thats the problem?
https://discussions.apple.com/thread/6627711
Otherwise airfoil could be worth a try.
@ExTra good luck with the email, I would have lost my shit and smashed something a long time ago