The phrase "up to one" was inadvertently omitted from Mass Mutiny's rules text. The card has received errata to correct this.

You can cast Mass Mutiny even if an opponent doesn't control any creatures. You simply won't choose a target for that opponent.

Mass Mutiny can target untapped creatures.

Gaining control of a creature doesn't cause you to gain control of any Auras or Equipment attached to it. However, if you gain control of a creature enchanted by an Aura with umbra armor and that creature would be destroyed that turn, instead the Aura will be destroyed and the creature will survive.