Nope, it caps out at 1/4, so being under ten forcefields is the same as being under one for firepower purposes.
Yes, Military Command Station damage is affected by having a FF over it, but its main advantage is not its firepower, but is its translocation effect.
As to your last question: the enemy troops are never having their firepower reduced by being under or near your force fields. And same for you when you're near theirs. And in terms of being near your own force fields, that affects nothing if the firing ships aren't actually under the force field.