Fun fact to be included in the rationalization of "The Galileo Seven": the leak left no puddle.
Seeing that it took place at the very bottom of the craft (and that the heroes checked from under the craft immediately after the crash!), we might want to rule out liquid fuel right there.
On the other hand, if the fuel is volatile or cryogenic in nature, it would make sense for it to escape in gaseous form - but also for a small amount of it to remain in the system as a liquid, completely useless because the loss of pressure in the system would mean it could not be pumped to where it was needed. It could be successfully vented in zero gee, though.
(Modify the nature of the fuel with the fact that its supply is measured in "pounds psi", a nonsensical unit today. But we may still speculate that anything involving "psi" is a gas under pressure. Or perhaps a telekinetic substance?)
When the craft takes off from the surface, it is forced to hover for the better part of a minute because the cavemen cling to it. This does not appreciably affect its ability to reach orbit later on, though - even though the heroes were extremely concerned about extra weight at an earlier stage. It makes sense, then, to assume that the hover is not part of the orbital ascent at all, but merely a preliminary maneuver performed by a separate drive system, an antigrav of some sort that doesn't need the lost fuel but does need energy of some other sort.
The phasers may have been crucial at that stage, then. Or then during the entire ascent, which for this once is performed using the antigravs rather than the normal engines - basically the equivalent of an F-16 trying to get from airbase to airbase by taxiing on a highway all the way.
That Scotty likens the phasers to a "substitute fuel supply" is probably our best proof that it is nothing of the sort. After all, Spock is convinced of the opposite, and if he says there's no substitute supply, we should trust his word. It takes Scotty's genius to see that the heroes can make do without a substitute fuel supply, and use the phasers instead of such a supply...
Timo Saloniemi