Even before freeing all the Staff's energy, Nathaniel is already seriously weakened. And remember Barti's the only one keeping the spirits from breaking the hell out of that staff, so the moment Nathaniel dismissed Barti the spirits' energies will all be released at once, and at such a close proximity even with the boots I doubt it if Nat's got any chance of survival.

he can. If, at the time he released barti he pull his leg up and think to go away. Then, when barti go the leg will touch the land and at that time he will be teleported into about 7 leagues away. But, it gets too many "IF", and the ending won't be as good as this one if it ends like that

I don't think he could have survived either. I don't think he could get away fast enough. I imagine he fainted instantaneously after dismissing Bartimaeus. He was already thoroughly wasted before Bartimaeus entered his body.