*Let me start off by saying I only use the MC6Pro for patch/program changes, so this may not be helpful for those with latency issues.*
*kpahuitsing replied as I was typing. His response was my initial problem. (charging cable vs data)*
I read where you tried several different cables, so this may all be useless?
For me, it was all about the cable. I couldn't get the A to C to do anything with my MC6Pro, even with a couple of different cables I had. I almost gave up until I read about so many people having success with the A to C. I decided to buy a cable from a music store instead of amazon, and it started working immediately. This is the cable I bought from Sweetwater - StarTech.com USB31AC1M USB-C to USB Type A Cable - 3 foot
Now my MC6Pro powers up and works great, but once again I ONLY use it for patch/program changes, not anything fancy (short press, long press, etc.)