Certainly Cassius thought he could cast a death curse and Harry felt it hit. So no magic feather there.It's certainly open to discussion on what it means. For me personally I take it as meaning that at the moments before his death that he will be alone fighting in some last ditch battle. But I am a romantic.
In terms of Cassius aging, his apparent age changed enough so that Harry did recognize him as who he was. So was it a magic feather???
But didn't Harry also note how old he looked? Cassius's may have aged, but he was still Cassius, it wouldn't have been hard to know who he was.. No, no magic feather for this Dumbo.
Well, considering Cassius himself was dying alone, and as bitter and pissed as he was because he felt it was unfair that he couldn't get another coin, he wanted that for Harry as well.. And if you consider at the end of Changes, yes, Harry heard his voice echoing in his head, but the truth of the matter was, Harry was only "mostly dead," he fell into Mab's arms, heard her murmur "hush," was dragged to Demonreach on life support and had his soul walk about. 1] He didn't die. 2] He wasn't alone.. Maybe a wizard would feel any curse thrown by someone dying, magical talent or not.. However unless it is a magical practitioner of some sort, there is very little sting in it.