It was obviously Darunia.
Darunia did not defeat Volvagia. Darunia himself tells Link that one of his ancestors was the one who defeated Volvagia originally.
Romani and Cremia are the Terminain counterparts of Malon, the adult Link-goro is the Terminain counterpart of Darunia's son who wasn't born yet, and Darmani is the Terminain counterpart of the goron who defeated Volvagia with the hammer.
I don't think the adult Goron named Link is the counterpart of the young Goron Link from Ocarina of Time. I always felt that the Goron Elder's young son (the one who wouldn't stop crying) was the Terminian counterpart, if only because they are the only infant Gorons seen in both games.
If Darmani is the counterpart to anyone from Ocarina of Time, it's most likely going to be Darunia, not an ancient ancestor we never even met or learnt the name of.
As for your point about Link the Goron not being born yet in Ocarina of Time, that just raises the question of how can there be a counterpart of someone who doesn't exist yet?
In the same way, there is a counterpart to Malon's older self, despite the fact Malon was still a young girl when Link entered Termina and Ingo's counterpart would be the Gorman brothers. Why would he have about 4 counterparts?
Maybe, just maybe, not everyone in Termina is truly a counterpart of someone from Hyrule. Link doesn't have one, Zelda doesn't have one, Ganondorf doesn't have one, the Happy Mask Salesman doesn't have one, the Zora King doesn't have one, Nabooru doesn't have one. Then there are all the people of Termina who don't have any link to people from Hyrule, like the Deku King, Princess and Butler, Kamaro, Kafei, Mayor Dotour and Madame Aroma, all of the Indigo-Gos except Lulu, the swordsmiths, Igos du Ikana and his guards, the beavers living in the Great Bay. There are no doubt more that I haven't mentioned.
My point is not everyone in Termina is a counterpart to someone from Hyrule. There are people who exclusively inhabit Termina, the same way there are people who exclusively inhabit Hyrule. It's likely that the adult Goron named Link is just there for the Anju & Kafei side-quest, since he shares nothing with the Goron Link from Ocarina of Time besides the name. All of the actual counterparts look exactly the same as their Hyrulian versions (some with minor differences), so an adult Goron being the counterpart to an infant Goron doesn't fit with the pattern found throughout the whole game.
