No the AAA’s ability to reduce aircraft attack power does not stack, but having two AAAs will allow you to reduce the attack value of up to 6 aircraft. Also, the controlling player (attacker) chooses SBR casualties.
From our limited experience, one fighter usually isn’t enough to deter a SBR. Remember that there are no longer any AA shots on attacking planes during a SBR so fighters are the only thing the defender has to inflict damage on the attacker. If he chooses not to intercept due to escorts, the attacker will inflict bombing damage with no losses. Escorts were used more to take hits from the defending fighters so the bombers would continue to do damage. A territory defended by an AA gun and a stack of fighters is a hard target and usually not cost effective to bomb unless you have a massive air force. Germany needs to use it’s full airforce if it wants to SBR England effectively. Russia needs to actually build fighters or ask for allied fighters if it wants to defend it’s factories.
We could increase the max damage a facility could take to increase the potential damage of a SBR. We’ll have to play more games with this to see how changes things.