Interesting discussion... even by half of you who were unnecessarily snotty.
I misread this line and was about to respond, "Hey, Brin has sort of earned the right to be snotty" :)
I thought your perspective was interesting. For me, even before ch113 and prior to (I think) seeing the idea on this subreddit, I really expected partial transfiguration to be The Solution (though the antimatter threat seemed somewhat silly to me). Consider: The prophesy strongly suggested that the dark lord's defeat would be a result of a power he knows not. Partial transfiguration was specifically pointed out on several occasions to be a power He Knows Not (e.g. by Dumbledore), and indeed Prof Q. didn't know about it. It's also rather overpowered (transfiguration in general is overpowered the 'cognitive limits' on it make it less so. With the stone at play we also have a heavy focus on transfiguration. There was also prior discussion of transfiguration in battle. etc.
I don't raise this to debate it, just point out some of the narrative reasons why a great many people expected it. But also, many people did not. My SO (who has not read this subreddit) was completely surprised by it and had been thinking more along your lines. Several other people here made simulation style arguments.
I would have expected talk his way out solutions to be dismissed as "magical" in the way you dismiss the use of partial transfiguration, because it's too easy to go "well I wouldn't be convinced by that gobblety gook". This might also be because I am not a writer and it wouldn't be in my power to craft such a turn in a way that as a reader I'd buy, but I completely believe other people could at least in a context where they control the whole story (after all, I've read a significant chunk of your fictional work, and I never recall finding the responses of the characters unbelievable.). That said, I don't think I would have preferred your solution to EY's but you also weren't writing the rest of the story, I've already seen EY's etc. Some of this may be context building.
One thought I have is that HPMOR doesn't build up the intellectual framework to justify a simulation argument (other than via some lesswrong fanservice like mentioning "timeless decision theory"), it might be reasonable to assume the reader is familiar with these concepts-- similar to how much of the "post-singularity scifi" depends on just accepting mind uploading or you'll toss the book on the 5th page-- but it's a little late to step up the meta level another step. Many of the readers in this subreddit are not actually strongly familiar with these ideas-- I think part of EY's goals with HPMOR has been to reach a more general audience, and as a result is prone to consider complex arguments like that a bunch of intellectual wankery unless very carefully introduced. We can see evidence in the bimodal distribution of people who thought everything after 109 was a simulation inside the mirror vs people who thought the former group were on drugs.