Here are some rulings that resulted from the most recent session of our DF campaign. Also, from afterward.
My hopefully unambiguous ruling is that missiles targeted at an area can't be subject to the spell. You can't return missiles that accidentally hit a target, or attempt to make yourself a target deliberately to return it. That way lies game-y silliness, in my opinion. It also means that the very existence of this relatively easy-to-get spell means missile fire against a group with a wizard, even against the ground near them, is extremely risky.
Epic Smash prereqs
Mo's player asked about Epic Smash, from DFD Barbarians. While this wasn't his question, I did note that the prereqs include Momentary Strength, which allows you to buy Power Blow, but doesn't explicitly mention buying Power Blow. As I intended it, you'd have at least a point in Power Blow before you could have Epic Smash.