1) In Diamond and Pearl, a Pokémon with No Guard can successfully hit an opponent's Pokémon that has used Protect or Detect, but only if the attacker uses a move with less than 100% accuracy; the lower the initial accuracy of the move is, the higher the chance of the move breaking through Protect/Detect. Thus, when used against Protect or Detect, moves like Swift and Aerial Ace will always be blocked. OHKO moves are exempted from this behavior; they will always miss. This was a bug, and was fixed in Platinum.
2) It may, may not.
3) Again, it may, may not.
4) Yes. Golurk, Machop, Machoke, Golett.
6) No. And Focus Energy does not guarantee Focus Punch a hit.