"Despite the additional fuel tanks fitted, more extra fuel was carried in 5 gallon cans."
I remember reading--recently--that one of the B-25 pilots saw a five-gallon can go past one of his props at 150 mph, having been emptied and tossed out of an aircraft ahead. If it had hit the prop, of course, he'd have had it.
Also, it wasn't a "merchant ship" that the Hornet encountered but a fishing boat.
And finally, remember that the "450 feet of deck available for takeoff" was available only to the very last B-25 to go. All of the others had increasingly less deck to work with, as they were parked farther forward, and
Doolittle himself--number one in line--had the shortest deck of all.