The 5mm shaft is much beefier and will help prevent snapping if too much lateral force is exerted on the shaft.
The UBEC is nice for several reasons:
1) It removes all the load off the internal BEC
2) Like you said, a UBEC reduces heat
3) A UBEC actually improves runtime slightly due to the switching nature.
4) Most internal BECs run at only 5v, limiting servo speed and power. Most UBECs are 6v.
5) Since the HV BEC runs off only
one of the packs, the pack connected to the internal BEC would be discharged deeper than the other one. A UBEC, being connected across both packs, discharges both packs equally.
Not sure if the HV ESC can even run on 5s, but if it does, I would use the 6.5 motor if anything.
Not sure about
your spontaneous combustion issue. It's not that common or proven. Or, do you mean your
vehicle's spontaneous combustion problem?
Anyway, unless you know what caused your issues, it's gonna be hard to say what will work to keep them from happening. As a start, I'd gear down more and watch temps closely. If everything runs cool under normal running, gear up slowly.