No. It is normal for legs occasionally to fall asleep with certain dramatic positions which cause compression of arteries, especially when the positions are held for long periods of time. If your legs are falling asleep every time you sit in a normal position, you should have your circulation evaluated.
Depends. It could be something as simple as crossing your legs when you sit down, as dr. Koslow said. Putting pressure on just the right spot can prevent you from walking normally for a few moments after standing. You can also lose all feeling all together for a few moments. On the other hand, if you develop terrible pain when sitting, or your leg turns pale, purple, and/or cold, you need to see a doctor.
No. You should see a neurologist to see if there is a nerve related reason for this.
Yes. Don't cross your legs on lean on your thighs.