1. Only if your electronics skills can support 2 modulator.
according to my experience,
if highest quality of electronics you can mess with is X then,
single X > primary(x/1.3) with secondary(x/1.3)
2. High Efficiency Energy Converters.
3. Melee attacks will always get through no matter what frequency you use. Using lower frequency will take off few damage off per hit, if the attack is melee.
I will be brutally honest.
Shields are only good for stopping bullets, don't expect them to block crossbow bolts or anything slower than that. You might as well as specialize it to block bullets.
Edit:, oh right, shields also block quiet number of psi attacks, so keep that in mind.