(The HPMOR discussion thread after this one is here.)
The previous thread is over the 500-comment threshold, so let's start a new Harry Potter and the Methods of Rationality discussion thread. This is the place to discuss Eliezer Yudkowsky's Harry Potter fanfic and anything related to it. The latest chapter as of 09/09/2011 is Ch. 77.
The first 5 discussion threads are on the main page under the harry_potter tag. Threads 6 and on (including this one) are in the discussion section using its separate tag system. Also: one, two, three, four, five, six, seven, eight. The fanfiction.net author page is the central location for information about updates and links to HPMOR-related goodies, and AdeleneDawner has kept an archive of Author's Notes.
As a reminder, it's often useful to start your comment by indicating which chapter you are commenting on.
Spoiler Warning: this thread is full of spoilers. With few exceptions, spoilers for MOR and canon are fair game to post, without warning or rot13. More specifically:
You do not need to rot13 anything about HP:MoR or the original Harry Potter series unless you are posting insider information from Eliezer Yudkowsky which is not supposed to be publicly available (which includes public statements by Eliezer that have been retracted).
If there is evidence for X in MOR and/or canon then it's fine to post about X without rot13, even if you also have heard privately from Eliezer that X is true. But you should not post that "Eliezer said X is true" unless you use rot13.
I'd been assuming that they'd abuse the existing rules--Ravenclaw and Slytherin would agree not to catch the Snitch until both teams have racked up an obscene amount of points, and then they'd deliberately engineer a tie. This would both force a rule change and allow both to win the Cup.
But you've pointed out a potential problem: the tie can be broken by any professor. All McGonagall has to do, once she realizes what's going on, is promise to break the tie in favor of one side, and the other side will have no choice but to defect. Quirrel's bound by his promise not to award house points unfairly, but maybe he could blackmail Snape into promising to maintain the tie.
Only as far as anyone sees. Quirrell is not Harry and is not opposed to lying.