Had he approached Padma in a friendly manner, putting himself on equal footing (instead of trying to teach and impress!), and then told her pretty much the same things he ghost-whispered, it would still have likely redeemed her, except he also would have gained a precious friend and possibly ally (and Hermione's respect).

The may be ok advice and perhaps worth a shot. It may even work - in a fantasy story. But real people tend to have better (or, rather, stronger) social and psychological boundaries - it is actually hard to exact fundamental personal change from people just by approaching them in a friendly manner. And giving unsolicited brutally personal advice to people actually isn't a reliable way to gain friends.

Interestingly, he would also have been following both Quirrell and Flitwick's advice in doing so.

Not Quirrel's. Not like that. Quirrel's advice pertained to an entirely different sort of influence than what you and Flitwick suggest. With Quirrel's Slytherin-typical strategy you influence by controlling the political, reputational payoffs. Direct heart-to-hearts are completely opposed to the spirit of it.

I also suggest that "self-centredness" is not the relevant flaw of Harry's here. This is actually a situation where more self-centredness would have prevented the err (such as it was). Harry has blurry boundaries on just what he is optimising for. Is he optimising for his self, is he optimising for blades of sentient grass or is he optimising for what Hermione might call "her own business"? People don't tend to like it when you act to control things that they don't perceive to be 'yours' - even if, as in this case, it is a benefit to all concerned. A self-centred Harry would have made entirely different mistakes to boundariless-Harry.