I would simply change it such that only the person reporting approach the ref so if the ref tells the defense a number or they hear a number other than who approached the ref, they can challenge that to ensure they get the right information.
The defense has no incentive to make such a challenge. If the ref tells them #70 is eligible, that's the only extra player they have to guard; if the offense throws it to #68, that's illegal touching. Open and shut.
Defenses are not going to guard additional ineligible players based on guesswork about who may have talked to the ref. They are going to guard the players that report, because those are the only players they have to guard. If the ref screws up and relays the wrong player, it's the offense that's punished, not the defense.