(4) Harry is deeply conflicted about his actions. Yet there's a case to be made that that Harry's decision making process (the one he's now feeling conflicted about) was better than Dumbledore's.

I think it was. Dumbledore was considering it a blackmail situation, which it clearly was not. Malfoy didn't want the money, he wanted revenge and punishment. Harry did not give in to blackmail, he found a way to save a friend against someone trying to kill them.

People may conclude that Harry can be pushed to extremes by attacking those he cares about. Sounds a lot like Malfoy. This tends to imply some susceptibility to blackmail, but it's not very strong evidence. I'd say it's stronger evidence that's it's dangerous to mess with his friends, but potentially useful to do so if you can direct the retribution to a target of your own selection.