I think your theory about Snape is awesome. The first line in the prophecy "The one with the power to vanquish the dark lord approaches..." could be referring to Snape. We know that Snape was the one who heard the prophecy as he stood outside the door listening. I am not going to make any guesses as to whom Snape would be protecting if/when he kills LV, but I don't think it is out of the question to consider Draco. We dont know if Draco was born late july, so he could fit there, Draco has been marked by LV as an equal according to your theory about the Dark Mark and his parents have also thrice defied LV. First by Lucius not seeking out LV after he lost his powers, then by Lucius again by not destroying Harry when he had the chance. The third time that Draco's parents defied LV was at the beginning of HBP when Narcissa made the Unbreakable Vow to Snape. It was not her place to interfere with LV's plans for Draco. In a round about way Draco fits the description of the one born from parents who thrice defied LV and we don't know whether Draco was born in late July or not. I am taking into account that JK Rowling like to throw curve balls in her stories and this could be another surprise for readers. I wholeheartedly support your theory about Snape, and will be extremely impressed if it turns out to be true.