imo opinion Turkey should act now and screw searching for a 'legitimate' reason. ISIL themselves are terrorsits and they threatened Turkey before, so i think if Turkey goes in without firing rockets in its own soil or shooting at the tomb, i believe the Turkish people's opinion won't be affected much by this issue.plus we would still have some element of surprise.
But if Turkey waits and waits till ISIL shoots at the tomb, then Turkish people might doubt immediately. plus ISIL will have enough time to anticipate.