The one-battle per turn rule does not apply to side battles, for the obvious reason that Idle Gossip and CSA would never work otherwise. The other Heroes are legal targets for banding into the side battle, just as long as they are able to come from their original location (i.e. if the enhancement said band from hand, the heroes could only be brought into the side battle by another banding enhancement that bands from hand, since that is where the heroes are until further notice). I am fairly certain that any heroes that were banded into a side battle that includes one of the original attackers will join the main battle if the heroes survive the battle, just as any enhancements played in such a side battle would transfer.
I have another question regarding CSA and banding. If my opponent bands in Heroes from their hand, and I play CSA... Can I choose one of those heroes, or do they return to hand until the interrupt finishes?
If the banding is the result of the last enhancement played by your opponent, then you cannot choose a banded hero, because the interrupt happens suspending the effect of the banding enhancement, so the banded hero is not a legal target for the rest of CSA. A similar situation occurs when you use Joseph in Prison following an opponent's Jehoiada's Strength. In this case, your opponent gets lucky with the ItB part of JiP, because JS is interrupted so only the initial attacker(s) is removed.
