Upload & Sell: On
1. Technically yes, but to do so you'd need to jailbreak your iPad and would need terminal access the drive such as with a Macbook (so it would kind of defeat the purpose because you could just plug the drive into the Macbook). Out of the box, the iPad cannot be plugged into an external hard drive except for a memory card via a Camera Connection Kit.
2. Technically yes, but you'd need to jailbreak the iPad, load a file viewer such as iFile to do so. Out of the box, the iPad does not have a user-accessible file structure
3. The iPad does not have an "air card", it has a 3G radio, which is the same radio in 3G phones. 3G speeds are around 30kb/sec download, 5kb/sec upload in my experience with ATT in the congested Washington DC market.
4. Yes, you can turn the 3G on and off on the iPad -- the iPad also has Wifi.
5. Technically yes, with a jailbreak - ATT disables tethering the iPhone to an iPad out of the box.