1 - It depends on the type of airplane and your flying style. If you're a beginner, or intermediate flier, it won't matter much. If you fly precision aerobetics, it will matter.

2 - You could try bending them by hand, but your results will be unpredictable.

3 - A mustang has a 4-blade prop. I don't know of anyone who makes a scale prop for a 40 size Mustang, but even if you found one, you wouldn't want to fly with it (It would only be used for static display) because when scaled down to the small size we use, a 4-blade prop becomes very inefficient.