Hi,
As Arvisrend mentioned, it is recommended to finish them because you will not be able to do it once you start the mission.
Are you sure you are not mistaking Melvin for Merzul? (some people did, and that's why the didn't get the quest).
If you're sure you're speaking to Melvin, maybe upload your savegame (upload attachment option is under the post when you write it) and Arvisrend can investigate what's wrong.