I would add a little tylose powder to my fondant or gumpaste to my fondant. This will allow it to dry firm. Roll out the fondant thick enough to insert 2 wooden skewers at least 1/2 way through. you want to insert the skewers while the fondant is still soft, gently smooth with your hands to ensure that the skewers are attached to the fondant. Lay flat todry at least overnight. The skewers will provide support and allow you to attach the dress to the cake. HTH
Such a cute cake...and for the necklace you can use white sugar pearls, if you want to keep everything edible. Attach with edible glue. The one in the pic looks like beads you use on fabric.
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