they could have tried that track with the Federation I suppose, but the TNG-era Federation tended to bend over backward to avoid any conflict, so I don't think they would have gone for tighter monitoring or a more assertive approach toward the Cardassians.
Just to be clear, I don't condone Federation behaviour in any way and I totally agree that political actions would have most likely not made the Feds monitor the DMZ better (which would have hardly been an affront to the Cardies).
But once the Maquis attacked both sides there was no way the Feds could have implemented a better policy even in theory, they first had to stop the violence of the Maquis.
This is my problem, whatever slim chances the settlers (besides doing the common-sensical thing and leaving their colonies) had, their ruined it once they fought against their own government. And once they went down this separationist path they basically became suicidal. Even the tiniest chance to change Federation policy via political means in the DMZ is better than this.