Afraid I don't know with the first question. It does sounds like the level of the Pokemon you get will depend on the number of badges you have, so maybe its better to wait to pick them up so long as they don't limit the number of Pokemon you have waiting to be transferred.
I remember reading somewhere with the second question that only 1 could be caught and transferred into a game of Tornadus, Thundurus, etc, and I assume the same holds for all the legendaries (since you can catch a few regular Pokemon too). So you can't go get extras for a friend or anything xp
The Radar tells you everything about transferring, but here's the rundown: you play the DR for a bit and get some cool stuff (I think it has ten slots and items stack), when you want the stuff you tell it to transfer and then in your games main menu you go to Unova Link and then it's the third option. After you have done that it transfers your items into your bag and your Pokemon to the box.
You can play with the Radar and get Pokemon before you start playing B/W2 so like I have now, before the second town, a team of Snivy, Drifloon, Munna, and Riolu. The DR ones came to me at level 5.