I don't see any fundamentalists jumping to allegory. If they do, they aren't fundamentalist any more.
My thought too.
As respects one point made by Leo: my observation is that fundy Christians who prefer to believe in the literal account of Adam and Eve are the ones I see jumping over to the 'allegorical' opinion when painted into the corner by scientific evidence. So my discussion is involving fundies who leap out of panic.
Okay so you are only referring to fundies who as a resort use the allegorical interpretation as a fall-back position. But my negative answer to your question "Wouldn't an allegorical Eden void the Ransom?" still stands. If they fall back on Eden as allegory, they could easily have a doctrine of the ransom that doesn't have anything to do with Eden. In fact, as argued above, there is no necessary reason to predicate a ransom doctrine on the Eden narrative at all.