I’ve concluded the PX4 firmware isn’t capable of controlling the tri-copter (keep it from spinning). Regardless of CH4 direction. Switched back to ArduCopter 3.4.6 - TriCopter and everything is well.
I am very happy with QGC. Once I flashed the firmware with MP I went back into QGC and used it to setup all parameters and perform all necessary sensor calibrations. Very happy with QGC in that regard. I decided to flash with Mission Planner as it wasn’t clear if I was going to get the TriCopter version of 3.4.6 if I flashed with QGC. But that’s not a biggy.
As a side note - just in case someone stumbles across this obscure post - I had a boatload of issues flashing the Pixhawk 2.4.8 as well as connecting to Mavlink (with either USB or STK Radios). Turned out my SD card in the Pixhawk was flaky. Pulled it out and reinserted and whalla - no issues flashing or connecting to Mavlink. Going to get a new one and pitch this one as it’s the second time I’ve run across cheapo SD cards that sort of work, sometimes, maybe.
I see a lot of posts in this forum go unanswered. So by replying to my own post at least I don’t get a big fat goose egg. 
Cheers.
Ed