1. No, you don't have superior cover. The enemy chooses any corner of its square and traces lines to all corners of your square. No matter which corner he chooses, he has unbroken line of sight to at least 2 corners of your square, which is only cover, not superior cover. Your DM may rule differently, but this is RAW.
2. Technically neither of you has superior cover (see #1) and can't make a stealth check unless you were previously hidden.
3. Assuming you somehow got stealth in D2 despite #1 and #2, you use deft strike and pick D3 or D4, doesn't matter both give the enemy cover... after the attack you can return to D2, but per #1 you don't have superior cover and can't stealth.
4. Opinions on FG differ. I personally rule that the power means to let you move at full speed without penalty to stealth; others think it means that you can make a check when you only have cover or concealment. That said, even in B3 you definitely don't have cover or concealment, so even if you pass the stealth check, you immediately become unhidden as the enemy has unbroken line of sight to you.
5. Please see the errata for Stealth. Stealth is now at the end of a move action, and you retain the benefits of stealth until you complete the action that causes you to lose stealth.