RE: Unexplained loss of control
I agree about sending the RX in. I would also send the TX as well. As far as heat goes, the R617FS has proven to be extremely robust when it comes to heat. I do not recall hearing about any issues with it, especially heat related one. Low voltage is not a likely cause either. The 617 will work to down below 3 volts, the point at which many servos will not move. Given the dual packs, I'm not voting for voltage in this case.
I am still very curious about exactly what happened. Did it just stop responding? Did it go into failsafe (low throttle)? And how was the BEC wired in?