The new FAQ has an update on leap. You can no longer interupt a leap attack in mid leap. You can use a guard, or trap before the leap (ie the originating space) of after the leap. They also clarified that leap uses up all remaining movement points. I would assume that with a hero, they could still use fatigue to do something like opening or closing a door, but it would have to be fatigue based as no regular movement points would be left after the leap was done. Additionaly, leap no longer lets you get out of a grapple, you can grapple a leaper before or after the leap, but not in mid leap. Overall this reduces the ability to react to leap to before the start of the leap or afterwords, not in the middle of the leap. Anything that could affect the leaper now has an effect (no more getting out of grapples by leaping over the grappler).
As far as AOE spells and leap interacting, my first thought is they should not be compatible. If you leap you get one attack on each figure over which you leap. The blast, or breath, etc would not then continue. That being said I can sort of see an argument for having the AOE take effect sort of like a broom sweep. The attack would then sweep an area to each side of the leap. This would be a thematic argument however, and I am usually leary about allowing anything like this. Think of it like this. The hero leaps while using a breath rune, the attack starts from the position that the hero started on, is oriented along the path of the leap and moves with the leap. at the far end the breath continues from where the hero landed. Each monster in the area would only get attacked once however.
Brian