You can sell the old CD-key to whoever that wants it.

They can borrow the game from a friend or download it online. The key is not lock to the game installation but to the game account.

Long duration payment = less option to quit spontaneous.

http://www.wow-pro.com for leveling guide.
http://www.wow-loot.com for loot drops recommendation.
http://www.wowwiki.com for guides on the class you're playing.

As for taking multiple mobs, it can be done provided that u know how to play the class well and wearing good gear.