The NRL couldn't force Melbourne to offload certain individuals but they should've gotten heavy cap penalties with all contracts becoming null and void in an attempt to force the clubs hand. If the players opt to re-sign with the club still then that is their choice. It's their career afterall.
However, any player who knowingly takes part in salary cap cheating should also be punished via either suspension or being deregistered. They are just as guilty as the club.
I suspect the NRL did not have any rules relating to player behaviour or responsibility (and probably still don't) so couldn't punish the players even if they could prove they were guilty, and TBH it is almost impossible to prove. Every player would just make up some story or lay claim to not knowing what any other was one so how would they know if the club was overall over the cap? The "gifts" again if there is no NRL player requirement to declare gifts given by clubs or sponsors then why would they care?
CEO: Sign these two contracts
Player: why two?
CEO: Just an admin thing
Player ok:
Player: what about this speedboat?
CEO: Gift from our sponsor
Player: Ok, thanks.
NRL: Did you know the club was over the cap and cheating?
Player: No, I only know what I was being paid
NRL: But you had two contracts
Player: Yes, is that not allowed?
NRL: No
Player: Best talk to the club then
NRL: What about the speedboat
Player: Told it was a gift from a sponsor
NRL: Club has to declare that
Player: Go talk to the club then