Being a small company,
Finrod & co. were naturally taken captive by
Sauron. I doubt that could happen if an army from
Nargothrond accompanied them.
It would of course be much easier to speculate if we knew the number of the populations of Nargothrond and Tol-in-Gaurhoth. It is my impression that Sauron's Isle was not hugely inhabited, but served more as a watch tower over
Beleriand, albeit it was held by
Morgoth's greatest servant and ferocious beasts. Nargothrond, however, would still have had a large population of
Elves, among them many Calaquendi. This, coupled with the fact that
Luthien and
Huan would eventually have arrived after
Beren, makes me think that eventually Sauron's Isle would have fallen. Furthermore, I think that even without Huan and Luthien, the army of Nargothrond led by Finrod and Beren would have been victorious - after an exhausting battle, but nonetheless victorious.
All in all, therefore, I am quite optimistic. But perhaps all the ruckus of the battle would have attracted agents and
armies of Morgoth to Tol Sirion much sooner, and Beren and Luthien's (and Huan's) path to
Angband would have been watched and their plan to wrest the Silmaril from Morgoth foiled.
Arvedui said:
A great battle where we probably would have encountered
Glaurung for the first time, perhaps?
Glaurung first appeared in the year 265 of the
First Age, Finrod was slain two hundred years later.