The thing sounds like a transitional thing (kek). For a little band in the forest, custom should be sufficient, since everyone is living the norms every day.
You only need a lawspeaker when disputes involve a lot of notables who don't necessarily see each other much and who also have a lot to lose (and fight over) if the judgement doesn't go their way. An authoritative and hopefully neutral source of oral tradition would give the thing's judgements moral heft. Next step is to write the precedents down, which I expect happened at some point. That may be a fruitful area of inquiry for you. When did Scandinavians start writing precedents down and when did the state administration start assuming the powers of the thing? Somewhere in there is where you move from custom to law.
The question of what is law vs what is custom ultimately comes down to semantics, but I would say that when agents of the state assume exclusive authority to make the judgement and apply the punishment - confiscation, violence, whatever - then it's law. Any similar actions that don't rely on the state, and it is custom.
So if your neighbour parks his truck on your lawn and you ask him to park his truck elsewhare, it's custom. If you call the cops and try to have him charged with trespassing, it's law. If he's intransigent and you have to gather your other neighbours together and everyone gets excited and you beat him to death with rakes, it's custom. If his wife calls the cops on you for murder, its law. If your neighbours chase her around with leaf blowers until she tells the cops he ran away and she made it all up and he's fine, it's custom.