Few things you have wrong here.
Assuming Blizzcon is the first weekend in either October or November, prepatch is active for roughly 4 weeks, historically speaking. There would be no overlap with Blizzcon. Blizzcon in October doesn't effect a December release. If it's November 6th and 7th then the pre-patch would likely come out November 10th with a release 4 weeks later on December 8th(maybe even differently with how they've done releases that haven't been on Tuesday). This causes zero overlap no matter how you look at it and it wouldn't effect the world first race either with how they've been delaying first raids several weeks after an expansions launch, the first raid would come out in January and there would be zero holiday overlap for the WF race.
As for why, specifically, it won't come out in September as you're claiming that would cause more issues with everything you're saying. The expansion release would overlap with Canadian Thanksgiving, the raid would overlap with American Thanksgiving, and then having several hundred guilds falling apart because of Christmas right after that. There's a reason they're delaying Ny'alotha until after Christmas season. As for Blizzcon MDI and Arena tournaments would be significantly more messed up with a September release because what platform would they compete on? The expansion platform that just came out a week prior for October Blizzcon or a month prior for a November Blizzcon. Ya that sounds much better
lol.
With an early December release it's safely in the middle of Thanksgiving and Christmas with plenty of time for them to support launch, it doesn't take 3 weeks for tech support to solve launch problems, this isn't 2004. Most technical launch issues(all major issues) are solved in the first week of release(again historically accurate based on WoD, Legion, and BFA).
And this would be different from a pre-patch mess for MDI/Blizzcon tournaments how? As I explained above this would be WORSE than a December release significantly.
They've tried many releases, WoD came out right before American Thanksgiving and they said this was awful because of how the raid release lined up(with a significantly lower participation rate and high guild turnover rate in Highmaul). Cataclysm came out December 7th and was perfectly fine technically, the only downfall was the raids weren't delayed until after Christmas, which messed up a lot of players because of Christmas and New Years. Historically the best expansions, technically(since that's really you're only actual claim), have come out in Summer or in December.