Well, a day or two late would be a bit disappointing, but it wouldn't be the end of the world by any means. It's good to know that there is a decent chance of it arriving on the 12th so long as I use the correct delivery method, so I will order from Amazon. Thanks.
To answer your question in the thread regarding Amazon Pre-Orders, I'd say they usually deliver your order on the release date if it is stated below the item "Want to receive this the day it comes out?" in the case of Pokemon X and Y on Amazon it does, so as long as you choose the correct delivery method which would be first class then I'm pretty certain it would arrive on time. They have a tendency of delivering items a little late, I've heard a few stories myself but from personal experineces with Video Games on Amazon I usually get them on the date, I did with The Sims 3 so I'm sure it will be the same for you!
If you're uncertain about it just either get the eShop copy or use a different website such as Gamestop. Hope this clears things up for you and apologies if its a little long!