RE: 2.4 glitching?
It had an Optokill, The engine kept running and the OP is not sure whether or not he had control due ot the "oh *****" nature of these events catching the pilot out. (we have pretty well all been there and those that haven't will be there one day - I feel for you mate, it is a bad feeling).
Failsafe would have killed the engine wouldn't it? No matter if it was caused by low voltage a glitch or any other cause if it went into failsafe, the optokill should have used its designed in failsafe and killed the engine. I would be looking at the connecters from the servo changeout or perhaps something a silly as a spark plug cap not being on the engine properly.
2.4 can suffer glitches, it is just a heck of a lot more rare than other systems.
Which switches were you using for your manual switches? I have seen (even some very good brands) of those have a connection issue inside the switch when vibrated (try it some time - hook up a meter or a scope to a switch ad see what happens when you wiggle the swtich actuator).
Which Servos and which battery packs were you using? IT is possible that you were above the voltage threshold for the receiver but below it for the servos.
One last question - did you have your cell phone with you at the time? (maybe you took it out your pocket at the crash site to take photos) - or did anyone else on the flight line have theirs with them? This is a contentious issue but there is a possibility of a problem from those.