After letting the car sit for a few weeks, I went out to the garage to try and tackle the problem again. I figured I'd put the key in the ignition and try to start it.

A quick turn of the key and it started up just fine. In neutral, idles find and but stumbles a bit between 1500-2500RPM and sounds like it's backfiring. I've given up trying to fix this myself and I'm taking it to my local German independent shop so they can fix it. I just don't have the time or patience to deal with the problem anymore.