It was always supposed to be a one way trip. It wasn't feasible to get close enough to Japan to make it the half-way point and China was just a little further on so a better option. Had they been able to launch at the half way point then I'd guess that the planes would return to land on deck one at a time and then be quickly pitched overboard to clear the deck for the next one.
IIRC, there was an early option to have the Mitchells turn south after striking Tokyo so the crews could bail out near waiting subs but this was shelved as too risky to the subs to stay in a fixed area within Japanese controlled water that long. Also, the risk of interception by the Imperial Japanese fighters would be much greater since the Mitchells would be flying close to the Home islands much longer.