So, this is what i think of: If you wz edit a mob's body attack to 0, then you hit it, assuming that it hasn't die, it touches you, you did not receive anything(damage or knockback or miss), like you are completely an invisible being, the system detected something's wrong, then a/b you.
AND HERE is my suggestion, fbg wont ban in beginner snail training nor pyramid pq, i don't know about the beginner snail part, but i found that mule account can be used with fbg without ban assuiming that he/she nver attack a single mob. The phenomenon rule in maplestory is that a mob will act aggressively on you only if you act agressively on them, this doesn't include jr.newtie and some bosses though. From the pyramid pq, mule account and mob's phenomenon part, i suggest that fbg can be used with any mobs WITHOUT A/B ONLY IF YOU CAN 100% 1-HIT KO THEM. As you 1-hit KOed them, they wont stand a chance to act agressive towards you, so you can avoid the system check. This states why pyramid pq won't a/b even fbg is being used

Note: When the mob touches you and you receive damage, that is not called aggressive, only if you upset a mob then it follows you, it's called aggressive.
Well i don't have proof though, because i did not test yet, this is only my suggestion. ANY SUGGESTION AND DISCUSSION ARE MOST WELCOME!!