You're right about Bjorn dueling Askeladd in order to die a warriors death. When I first saw Bjorn in episode 4, I always thought he'd be a character we see maybe a few times the rest of the season, never thought they'd make everyone feel a type of way about him. Leifs commitment to bringing back Thorfinn is pretty admirable, he'd been searching for 11 years out of his own good will, and it's implied in the manga that he had already purchased multiple slaves mistakenly believing them to be Thorfinn.