What in the S&S3 duel or other reference clearly indicates Brandon began only intending to humiliate Willoughby with a minor wound? If so why does he put sword to Willoughby’s throat after giving him a minor wound?

I think that having S&S3's Brandon pin Willoughby to the ground in this manner is a good indication that Brandon intended to fight to humiliate his opponent, but not kill him. It's as if Brandon is sending Willoughby the message that he, Brandon, could kill Willoughby if he wanted, but that it would not be worth the trouble. The look that Brandon gives Willoughby before walking away from him seems like an expression of contempt, IMO. Andrew Davies wanted the duel to be a sword duel (presumably because it is more cinematic), but because a sword duel would not end until blood was drawn, and Brandon obviously does not kill Willoughby in the novel, Davies had to invent a way for Brandon to defeat Willoughby (through wounding him) while also sparing his life. This scene in S&S3 works quite well in that respect, IMO.

Where in the S&S Screenplay & Diaries does ET give her views on novel Brandon’s intent going into the duel or that her Brandon would feel absolutely the same?

She doesn't explicitly state her views. That is just my interpretation of some of the comments she makes, and the line that she included in the original screenplay.

Unfortunately there is no duel in S&S2 to scrutinize for Brandon’s intent as we can with S&S3.

True, and the dialogue in the original screenplay for S&S2 that referenced the duel didn't even make it into the final film. Does anyone else find it utterly baffling that this dialogue was cut? S&S2's complete removal of any references to the duel, and its alteration of Brandon's backstory to remove any mention of his attempted elopement with Eliza, change Brandon's character drastically, IMO. Jane Austen intended the character to be a man of hidden passion and ardor, but because the duel and the elopement are not in the final film version of S&S2, Rickman's Brandon lacks passion.

I do not think there is enough information to know ET’s views of the duel in the novel or what the screenplay’s Brandon would have intended.