So you have downloaded it? Because it seems like it was taken of the play store, but if someone has downloaded it, they could upload the apk somewhere else.
My friend ordered from Eachmall, he got it in less than 2 weeks, but I believe it came later than it said it would.
I cant help you with payment method, sorry.
This is probably your best bet for now, I believe Yoshi said something about getting it to mirror your computer screen eventually
http://gbatemp.net/threads/3ds-controller.375443/
So my setup is exactly the same except Windows 8 and a different IP. YoshiInAVoid said that they would be putting a debug code into the next version, so I'd say just wait for that and try when it's released. That's what I'm doing, anyway.
1. Yes you can update it later, with your 3ds.
2. You will be able to pirate the dlc (I believe) but you can't keep your save, unless there are new developments in the 3DS scene between now and the release.
3. As of right now its all speculation (since the closest to official confirmation is the...