Like I said, I'm willing to allow anything a player can reasonably justify, but do you think it might be fair to introduce some kinds of modifiers to the evocation difficulties when one element might be more or less appropriate in a given circumstance?
That is basically the idea of aspects as others have stated before me. If you want fire to be more effective against a target, declare something like "flammable clothes" aspect on them and tag it as you cast your spell. Want to give an air evocation an extra oomph? There might be "small debris" between yourself and your enemy that can be tagged to inflict additional damage when it is blown at your enemy's face.
It helps to think of the elements not only as those specific elements but mostly as the concepts behind them as described in YS.
You might not be able to declare anything earthy on a plane, but you sure as hell would be able to raise gravity around it with an earth evocation. It would be easier to do with an air evocation though, because you could declare a few aspects beforehand (there is after all a lot of air up there).
or insist on a REALLY good explanation for how this is supposed to work?
This is the idea behind the whole FATE system, I think. It does not try so much to detail realism as it creates a frame for really cool stories. As a matter of fact, the more unusual the application of an element is, the more fun it will probably be. Like using fire to create a sheet of ice to escape on, the kind of thing that will most likely only be usable once, because it depends entirely on the circumstances of the scene.
If you think there is a good reason that one element should be in an advantage over another, put an aspect on the scene to reflect that. In a damp environment, a pure fire block might not hold for long, so you can compel that aspect if anyone tries to do that. On the other hand, if the same wizard puts up a fire block invoking the "damp" aspect, he might argue, that the fire of his block might not last very long, but all that water will produce a whole lot of steam, making it impossible to follow them, and it might even block sight in addition. The same aspect used in totally different ways can get you totally different outcomes. The only difference is how creative anyone can get in using them to their advantage.