Success. The problem turned out to be a short between the mic audio line and ground. Sunil from amateurradiokits.in had sent me a second front panel PCB after I reported my initial problem to him. (His customer services has been awesome and very responsive.)
I could not figure out where the short was - so I wired up a the new PCB - using a smaller iron tip and smaller gauge solder. No short this time. I can report that the Uniden/Cobra mic with the 4 pin connector works fine - completely unmodified.
Here is a video of the completed rig receiving (sans the send/receive LED which I still have to wire up)
Thanks everyone for the help and advice.