You are mixing the job they are supposed to do and acts what Zoro did and how is he described XD Robin is on the ship to decipher the poneglyphs same how Nami is there to navigate that has absolutely nothing to do with being first mate...that's just their job.As far as I know Zoro is nothing officialy on the ship that's one of the reason why Oda is definitely pushing Zoro towards the first mate direction.Marco wasn't unofficial. The fact that he took the crew after Whitebeard's dead was what officialized his status as the First Mate. Zoro doesn't have anything like that.
Also, unofficial first mate is a meaningless title. That's like me saying Robin is the unofficial first mate because she gives Luffy information about the world.