If Ward leaves play, becomes unattached, or gets blanked, its constant ability ceases to apply, and control of the character reverts back to its previous owner (unless another "take control" effect is in force).
On the other hand, "take control" effects created by triggered abilities (Take the Black, Yoren, Bronn, Euron Greyjoy, Dagmer Cleftjaw) are independent of the source of the ability, and continue to apply even if the card that generated them leaves play.
Taking control of a card does NOT give you control of its attachments or duplicates. They remain attached, but you don't control them. They still provide their passive effects to the attached character, but you cannot use their triggered abilities. You cannot use a dupe to save the Warded character, but the dupe's owner can.
You cannot marshal or put into play duplicates of a Warded character - in order to attach a dupe you need to own and control both the dupe and the card being duped.
While you control another player's unique card, that player cannot marshal or put into play another copy of that card. If you control a unique character, or if a unique character is in your dead pile, you cannot take control of your opponent's copy of that unique character. RRG pg 22