The Black House isn't 100% in the pure-blood faction?
What HPMOR fact am I forgetting?
"This explains why it is so important to shoot missionaries on sight."
So, this is the single change that makes this story an AU?
From chapter 1:
This is not a strict single-point-of-departure fic - There exists a primary point of departure, at some point in the past, but also other alterations. The best term I've heard for this fic is "parallel universe."
Above is the Doyalist reason, and almost certainly the root reason.
The Watsonian reason was that it would force Voldemort to waste an hour in the preparation, making any attempt to steal the Stone take an hour longer. As traps go, it's reasonably clever.
The potion did serve its in-story purpose of banishing the flames blocking the doorway, after all, so it's not like Voldemort spent that time on the potion and then used his wand to take care of the flames.
The doom-sense could have a lot to do with both the strength of the magic and the degree to which it inherently clashes with the senser's inclinations or goals.
To me, it seemed that it scaled with their current mindsets and actions, with stronger being the more they contrasted.
Consider how strongly it was felt when Harry's "anti-death-ness" was particularly strong (e.g. after Harry killed his first Dementor) or when Voldemort's desecration of death was higher than normal (e.g. when he makes Inferi).
Inversely, look at weak the feeling is when their current mindsets are not so opposed (e.g. when Quirrell was in zombie-mode in ch 49, because he practically lacked a mind at those times). Though, the zombie-mode in ch 26 was still fairly strong, which could imply that zombie-mode isn't as big of a determinant as I initially suspected. It might well be another piece of evidence towards the importance mindset, however, as that ride ends with Quirrell leaving to "set something into motion," which is certainly ominous coming from Lord Voldemort and may well have been something Harry was inherently set against. I don't remember if we found/figured out what he was doing there.
Taking this into account, the lack of doom-sense during the transfiguration would simply be there not being much of a mindset left.
Though I fear I may have strayed from the original thrust of your post.
I can see him believing that it would be very difficult to get them to agree regardless.
Though, given the Drought of Living Death...
Naw, they still wouldn't want to leave him in Harry's care. Plus, then they might get all of his magical secrets (including Harry's (soul/mind's) magical heritage, rather than leaving them for Harry to use. Or they might Obliviate everything, depending on whether their paranoia wins over their greed.
But then there wouldn't be continued speculation that this is still the bad end.
Yes in HPMOR
Chapter 79:
“Born the 26th of September, 1955, to Quondia Quirrell, of an acknowledged tryst with Lirinus Lumblung...” intoned the Auror. “Sorted into Ravenclaw..."
The disparity is one of the reasons that the Aurors are sure he's not actually Quirinus Quirrell.
Ah. Even ignoring that the context is from when they are children, I don't consider the black sheep disowned from the family as having a bearing on where the House stands, but alright. Semantics, I suppose.