"Maybe US (and other powerful nations) can stealthily fund the locals and arm them to face the radicals"
That sort of activity usually ends with local radicals having externally find arms. Also, it hardly counts as change from inside when you armed one fraction of the conflict giving it advantage.
Innocent locals tend to be the ones who don't want guns, sadly. Or are not experienced with them.
Maybe US (and other powerful nations) can stealthily fund the locals and arm them to face the radicals, so that the change comes from within rather than outside. Again the problem lies in identifying the innocent locals from the radicals.
At this stage, I'm not even sure if there is a solution for syrian crisis.