I think you're spot on. I think that it is also Thorin's grief at play. He is
not only trying to regain his heritage and pride, he's trying to claim back the
symbol of his father.
He's lost his father and grandfather over and over again, it seems. First, to
the Dragon (miraculous recover thanks to back door), then to madness and
pursuit of the ring and so on. I know I'm attributing a lot of the LOTR and
appendices mythology here, but I do think that Thorin just can't stand to lose
this connection to his forefathers again.
I suspect he's made the oath because he doesn't really believe any of the
Dwarves or Bilbo to have taken
it.
"...the same behaviour or even the same good can appear distinguished to one person, pretentious to someone else, and cheap or showy to yet another." Bourdieu