I do agree with what you have added about contrasts.

As to the necklace, I knew that was shaky ground, so I will not add anything there.

But as to the generosity of Henry's efforts in getting William promoted I am more confident. I don't see evidence in the text that he did it just to create a sense of obligation in Fanny. If I've missed it, I'd appreciate if someone could point it out to me. If it's merely working backwards from the fact the he proposed immediately after dropping the news, that the whole must have been a plot to manipulate Fanny, I have to wonder. Don't you think with his his inflated ego, he thinks he can make her love him without these extreme measures?

When I read his telling Fanny the news, it appears he's is a man who is extremely happy, and I can imagine that it is a new feeling for him to do something kind for someone beyond himself. I know he's a good actor but I see sincerity in the telling. He was described as overjoyed to get the opportunity. He spoke with "unfeigned eagerness."
I would not allow myself yesterday to say how delighted, or to repeat half that the Admiral said in his praise. I deferred it all till his praise should be proved the praise of a friend, as this day does prove it. Now I may say that even I could not require William Price to excite a greater interest, or be followed by warmer wishes and higher commendation, than were most voluntarily bestowed by my uncle after the evening they had passed together."

It appears to me his two fold motives were because he liked William and wanted to make Fanny happy.

I can't quite compare this with Darcy's hiding his helping of Lydia. The Gardiners were forced to take the credit. How could Henry have hidden his interest? He gave William the lift, the Admiral's letters were addressed to him. For Henry to hide his effort would also require him to hide the Admiral's. William would not wish to hide the help he'd been given. If Henry's name were out of it, but Fanny knew that it was the Admiral she would have still associated it with Henry anyway.

I don't understand the need to deprecate the generosity because it wasn't "hard" for Henry to do. Sir Thomas has been generous to the Norrises and the Prices without going to extreme effort and they know he has done it. Yet we still see it as generosity.

I chuckled when I read your post because if you deny me this one sign that there could be a glimmer of goodness in this man, then he becomes only a "baddie" without any capacity or potential to move beyond his morally suspect character. :)