I still have trouble making sense of his declaration that he’ll take her by force.

I seem to have taken this much differently than everyone else, lol. The Sheriff hasn't made any bones about wanting to disrupt whatever schmoopy emotions Guy seems to have for Marian... mostly because the relationship that he and Guy have built relies on Guy putting him above everyone else and Guy's feelings towards Marian put that in jeopardy. Guy knows that. He also knows the Sheriff's tendency to, you know, kill people. I saw him telling the Sheriff that he'd take Marian by force as the only way he could think of to get the Sheriff to once again spare Marian's life.... and he wouldn't feel threatened because it would appear as if Guy no longer had the schmoopies for her. And bonus, he'd finally be able to protect her the way he's wanted to, and surely she'd be grateful that he saved her life, despite everything.

But maybe that's just me. :p