Hm.

On the one hand, this is a plausible enough theory, with few enough moving parts that I can legitimately assign it a non-negligible probability.

On the other... from a narrative perspective, this doesn't feel satisfying. It's a violation of Knox's First - we're bringing barely-mentioned and undeveloped characters in at the last minute and giving them primary roles in the plot.

Well, I suppose if the point is that Quirrell gets taken down by adults and Harry can solve Death in peace, that's valid...