It does not use CB or DS.
It uses Attack Speed.
Yes, physical pierce will work
25% chance to knockback
It should do the same damage from any distance, although it may hit the target more than once the bigger the target is.
Amplify Damage, or anything that would typically increase physical damage will help it. Fanaticism will make you cast it faster as well.
Yes, skill levels is the best stat for seismic leap.