On the other hand, the S&S2 filmmakers unknowingly put themselves into a potentially difficult position by having Rickman’s Brandon say that he thinks Willoughby’s intentions were “honorable” and that Willoughby intended to marry Marianne.

Would Brandon from the novel ever make this sort of statement? Well, in Chapter 31, we have Brandon saying of Willoughby: "His character is now before you -- expensive, dissipated, and worse than both. Knowing all this, as I have now known it many weeks, guess what I must have felt on seeing your sister as fond of him as ever, and on being assured that she was to marry him; guess what I must have felt for all your sakes. When I came to you last week and found you alone, I came determined to know the truth, though irresolute what to do when it was known. My behaviour must have seemed strange to you then; but now you will comprehend it. To suffer you all to be so deceived; to see your sister -- but what could I do? I had no hope of interfering with success; and sometimes I thought your sister's influence might yet reclaim him. But now, after such dishonourable usage, who can tell what were his designs on her?”

What a contrast! “But now, after such dishonourable usage, who can tell what were his designs on her?” – this line makes it clear that Austen’s Brandon does not think it likely that Willoughby ever had good intentions, and is certainly not willing to give him the benefit of the doubt. And why should he? After Willoughby so casually and callously ruins and abandons Brandon’s ward, Brandon has no reason to think well of him. I think it would be almost tantamount to insulting Eliza II and Marianne if Austen's Brandon DID opine that any of Willoughby’s “intentions were honorable.”