I touched on this before but I think he is too arrogant to think he must "attempt to purchase her good opinion". When he talks to Mary in ch. 31 you can see that he expects she will be his wife. In fact it's almost a Darcy kind of arrogance which assumes that they have so much to offer it doesn't really enter their mind that they would be refused. With Mr. Crawford's vanity, he thinks it is only modesty that holds her back from accepting him.

I have, I flatter myself, made no inconsiderable progress in her affections; but my own are entirely fixed.”
...
“For—for very little more than opportunity. Mary, she is not like her cousins; but I think I shall not ask in vain.”

I doubt that he thinks he needs this ace up his sleeve of William's promotion. I notice too he does not mention it to Fanny again as far as we know. Even during his creepy boxing her in and grilling her he doesn't resort to it.
ch. 33 He had vanity, which strongly inclined him in the first place to think she did love him, though she might not know it herself; and which, secondly, when constrained at last to admit that she did know her own present feelings, convinced him that he should be able in time to make those feelings what he wished...
Love such as his, in a man like himself, must with perseverance secure a return.

I agree that overhearing William and Fanny lament over his lack of opportunities is the impetus for Henry to get this idea. I also agree that if there were no Fanny he probably wouldn't have been inspired to do it. I already mentioned that his motives were two-fold. But neither of those things prove that it was not a generous impulse that inspired him to act.

Rachel asked if there were a possible more charitable view of Henry, and I think that Henry can be motivated by a generous impulse as shown by Williams promotion. This is not an attempt to say that it cancels out everything that is bad about him, nor what is ungentlemanly in his pursuit of Fanny.

Perhaps by saying a true spirit of generosity, I implied too much. Anonymous charity is wonderful, but perhaps the truest spirit of generosity is when the giver gives more than they can afford to give. I don't believe I ever used the word disinterested to describe it. It's funny you say "self-interest of marital bliss". Everybody is interested in that aren't they?

I agree completely with your sentence "I believe Henry did think to make Fanny happy with William’s promotion and it is very generous but it is unlikely he forgot it would also secure to him a great deal of gratitude."
It's funny that the word gratitude comes up time and again. Gratitude is not an uncommon theme in Austen's novels. There is some gratitude in Elizabeth Bennett, Henry Tilney, Marianne Dashwood in their relationships. Edmund lectures Fanny about gratitude, and Mary (without knowing of William's promotion) assumes that even if Fanny doesn't love Henry yet, her The gentleness and gratitude of her disposition would secure her all your own immediately.... ask her to love you, and she will never have the heart to refuse.” Everyone else seems to assume that about Fanny too. But Fanny with her "disgusting" spirit of independence does refuse, and I cheer her for it. Woo hoo Fanny!

Quoting you again
"Perhaps Henry’s learning how to modify his obtrusive attentions by ‘adapting… more and more to the gentleness and delicacy of her [Fanny’s] character’ (24) is a hopeful quality rather than (sorry) the relentless adaptability of a hunter in pursuit of his prey. (;D) "