Actually, I agree. Any jealousy is very little supported by text - the one exception being Mr.Darcy's increase in emotion when Elizabeth speaks of Wickham.

I can not really back up much of Darcy's innermost feelings, anyway, but jealousy of his cousin is less supported even than his gentlemanly-self-image, or his passion for Elizabeth. Your points about Darcy and Fitzwilliam's intimacy and mutual knowledge are telling.

I would hazard that Darcy did not tell Fitzwilliam (awkward that they both have the same name!) or anyone else what he was planning, because, frankly, I think he had only just decided to propose that night, or had not decided until he saw her. He would certainly have acted differently had he made up his mind prior to that evening, and Author Austen shows us no transition between the attracted-but-aloof Darcy and the decided-on-a-course-of-action Darcy until the moment of proposal.