Success! I realize this is going to sound terrible but I cross-threaded the nut. I laid it on the bolt slightly at an angle and started twisting with my fingers until it got stuck. I then carefully applied downward pressure while twisting the nut (with pliers) and the bolt's threads basically re-tapped the soft aluminum nut. I checked frequently to make sure I wasn't damaging the bolt at all and all went smoothly.

I find it odd that they would use such soft metal for a piece that holds together the entire shifter assembly. When riding you tend to put a lot of pressure on those shift levers so I'd think you'd want something a little more sturdy...

Anywho, the bike is back to full function. If I ever upgrade these shifters I'll know what kind of quality checks to look for...

Thanks!!