do we have any indication that finarfin was the family conflict mender? because his thing seems to be avoiding conflicts, not settling them: he trades tirion for alqualondë, he doesn't show up to the council of finwë - and after that, it's not him who reaches out to fingolfin, it's fingolfin who seeks him out. even during the Flight, his attempt to calm down the Noldor is futile and is only mentioned, without any real consequences. He doesn't confront Feanor directly after the Doom (at least it's not mentioned), and his return is his first proactive action, where he chooses something that conflicts directly with both of his brothers and it bears real consequences.
Besides, if Shibboleth implies Finarfin is the youngest child of Finwe, it doesn't really make sense for him to be the conflict mender (why would anyone listen to a kid?), but it does make sense if he's the one who's let out from the conflict, the one who chooses to step back (alqualondë!!!) and the one who disappears from the narrative (araman!!!)