And also for the surviving orcs to return back, should the information of the attack's failure be needed to reach Angband for any possible narrative purpose. (Retuns taking as much as the journey there is often forgotten in movies, to my experience. A common timeline continuity error which Tolkien put much effort into avoiding in his books. Not that all of a return must be shown, but it cannot be shown to be immediate.) E.g. should it be decided, that Boldog does survive the battle - for whateve reason - he should have been away from Angband for over one month and a half and as such in need of being briefed about what the current situation in Angband is.Correct, the Girdle of Melian will prevent any attack from penetrating into Doriath. Presumably this attack is an attempt to figure out how strong the defenses are. So, from the villains' perspective, it's more recon for a future attack, rather than an earnest effort to invade Doriath.
As far as the timing goes, the story takes place over multiple years. I'm not worried about finding a 3 week window to move troops from Angband to the borders of Doriath.
Communist times' bad quality road paving seems in style for an orc road imho: