1)
She is explaining that the cause of Ron's behavior is not what Harry thinks. そういうこと refers to Harry's thought; Ron thinks that Harry wrote his name(and put it into the Goblet).
2)
Yes, that's ~だなんて. This can be attached also to verbs/adjectives.
1) Could you explain the role of って here? I was previously thinking that って was quoting, but now it seems to me that this could hardly follow なく, and that なくって is more likely emphatic なくて. Is that correct?
1. I think it's as you said (なくっての意味 - 国語辞書 - goo辞書). Not sure if it would make sense if she was quoting Ron after saying that she thinks "that's not it", i.e., if Ron told her, she would probably be sure about it in the first place, not just think so.
2. Yeah. だなんて is fine to use (even after adj. and verbs, like Toritoribeさん said) when quoting someone, like in your example sentence
2)
Exactly. In the examples in the dictionary linked below, 本気にするだなんてばかね is valid whereas 手伝いだなんてできるか is not valid since the latter is not a quotation. なんて[副助]の意味 - 国語辞書 - goo辞書