Situation: Blood Ape leaps over a melee character. Melee character uses a Guard order to attack the blood ape mid-leap.
1. Can a guard order be used to make an interrupt attack on a leaping creature in mid-leap?
No. From the FAQ (you have a copy, right?):
Q: When making a Leap attack, is the leaping figure subject to Guard attacks, attacks produced by abilities such as Alertness, and Aura? If the figure is damaged by any of those effects, may it apply its Berserk ability to its attack roll (assuming it has Berserk)?
A: Yes on most counts. Leaping figures are immune to Aura, but otherwise subject to all the above noted effects. It may apply its Berserk ability if it takes damage during its leap. Note that if the figure is slain by any of these effects, then it may not make an attack roll at all. Guard may only be activated before or after a Leap attack is performed, never during.
2. Does it matter whether the leaping creature is attacking that character during the leap?
Q: Is it possible to Leap without doing a Leap attack? The AoD rules state that doing an attack roll is not mandatory with a Leap; if a figure does not roll attack dice, does it count as not attacking, and can said figure then make a normal attack?
A: The figure may perform the leap movement without making an attack. At the end of the movement, the figure may make one normal attack against any models it could normally target.
3. If the interrupt attack can be made, what happens if the character has the knockback ability? Which space is the blood ape knocked back from? Does the blood ape's attack still take place?
N/A since the leap may not be interrupted.
So, basically, the hero must use the guard order before the leap. Once the leap starts, the leap action cannot be interrupted.