And I'm not sure I can say exactly why ...

I agree that IT is a full participant in the flirtation, and perhaps even instigated it. But I still believe that it is not an honorable act of his to so heavily flirt with an engaged woman. And their half-whispered conversation at the Pump Room seems almost sinister to me. "What - always to be watched?" I have the impression that he is pressing for more than a mere flirtation here. He wants more than banter and a dance. And just because a woman may be willing, does not make it right. Or is that a Guy Thing? ;-)

I take Henry's remarks about "He knows what he is about" to be fairly cynical, that he's seen him put the moves on women before and knows he is just playing with her. Again, not very honorable behavior.

] I feel sorry for all the Tilney children. I think they all suffer under their father's roof. Even the eldest found a profession to get away from it. I see no evidence that the three siblings do not get along. Quite the opposite. From Chapter 7, we hear from Eleanor that "both my brothers are very affectionate"

I read this differently than you did. There is a bit before that that you left out:

"I have no sister, you know--and though Henry--though my brothers are very affectionate..."

I thought it telling that Eleanor started to say "and though Henry is very affectionate" but changed it to "my brothers" as if she recollected to herself that she has two brothers and ought to include Frederick.

Eleanor and Henry are very close, but I do not see the same with Frederick. Of course, he is much older, he's been in the army, she's been at home, it is not wonderful that they are not very close...