I was able to determine that for as of yet unknown reasons, my RP Gimbal was (apparently) drawing too much current from my system, which was the cause of my uncontrolled descents. Without getting into too many details, I'll just say that my P2V was able to fly at 15% charge with the RP/camera attached and the gimbal power cable unplugged. Plugging in the power cable prevented the motors from spooling up fast enough to take off once I was below 30% battery, and appears to be the source of my unexpected loss of thrust while flying.
I just wanted to pass along what I hope will be a positive experience with Rotorpixel support.
Upon completing my tests, I sent an email to Keri via the RP contact form. Within 10 minutes, I received a reply from Keri. I am a professional photographer and I happen to be leaving for a two-week photo safari on Saturday, and I mentioned this to him. He not only is sending me a replacement gimbal, but he is shipping via expedited 2-day post to ensure (customs notwithstanding) that I receive it in time for my trip.
My hope, of course, is that I simply had a defective unit. Considering that most people here are pleased with their setup, I'm optimistic.
So thank you to Keri for being so responsive to your customer.
-Jason