RE: Motor magnets exploding
Definitely overspeed. 5400Kv on 12.6 Volts is 68,040 RPM. I've never met a motor that could take more than about 30,000 RPM. Even under load, you're still going to be well over 30,000 RPM.
I'm pretty sure the specifications for these motors limit cell count to 8 NiCd/NiMH. That's 2S LiPoly.