1. Because SFC OP is based on SFB, the AMD's are based on ADD's of SFB. ADD's were strictly designed to destroy fighters and missile/drones, they just don't have the "punch" to really damage ships of size class 5 or larger. As a matter of fact, if I'm not mistaken, I thought I read once in SFB that explained ADD's as actually advanced forms of cannons that fire projectiles, but It has been a long time and I may be wrong on this.
2. I highly doubt it. I have notice this aswell, and Activision/Taldern might have thought about adding in code to allow this, but to my knowlege I have not seen any SFC OP weapons in SFC3 or vis versa.
3. I thought I might have heard one more weapon/feature, but its been so long now, I can't remember. Not too many people talk about this with OP anymore.