Fair enough.

I must admit, this makes my theory less likely, but I still don't see your reading as the unambiguously correct interpretation, but I will freely cede that it look plausible that it is an interrupt, not an elaboration. This may, in part, stem from the fact that I am a big proponent of using "-" in my writing, and my usage is somewhat nonstandard.

Even if that is right, I don't think it rules out my guess about Quirrell's plan, but again, I'm significantly less confident now.