Fumble doesn’t target the creature that it attaches Auras and Equipment to. You can attach Auras and Equipment to creatures you can’t target, such as an opponent’s creature with hexproof.

The Auras and Equipment must be attached to a creature if possible, even if the only creature they can all be attached to is an opponent’s creature.

The Auras and Equipment that you gain control of must all be attached to one creature that they can all legally be attached to. If they can’t all legally be attached to one creature, they all remain on the battlefield unattached (and Auras among them will be put into their owners’ graveyards).