eShop ROMs had already been out a while, I too believe there's a strong possibility it was ready beforehand and the "it took just one day" is as much a publicity stunt as the "trustworthy and dedicated beta testers" thing.
I don't know, but perhaps the "disable those sig checks" was a good hint? Let's not forget that they have been working on the 3DS for a long time. Also, the fact that the new card already supported them says something as well.