1. I'm not sure, sadly. I kinda hope not, since I just got a new 3DSXL..
2. It seems like audio will be English only, which is kind of sad. I was really looking forward to hearing Gin-san's voice.
3-5. When you transfer data between 3ds systems, it does a "system transfer". Everything that is on the system itself (friend codes, application data, etc) is transferred, erasing everything that was on the system before, and deleting it from the old one. So no, you can't pick and choose which data to send. And yes, if you bought a 3DS with Fire Emblem on it (are they really doing that?) and then transferred your old system data, it would delete Fire Emblem. As I understand it, anyway.
6. If you want to use a different SD card, you have to manually copy your SD card data using a PC. The SD cards are locked to work with only one system, you see, but the system transfer doesn't transfer that data.
It seems kind of confusing at first, but it's not as complicated as it sounds. Nintendo has some pretty good guides, and I'm sure people here would help with technical questions, if they can.