Creatures and lands you control untap at the same time as the active player's permanents. You can't choose to not untap them at that time.

Effects that state a creature or land you control doesn't untap during your untap step won't apply during another player's untap step.

Controlling more than one Prophet of Kruphix doesn't allow you to untap any creature or land more than once during a single untap step.

If you're casting a creature spell with bestow, whether or not it has flash will depend on if you're casting it as a creature or as an Aura. Prophet of Kruphix won't give flash to a spell with bestow you're casting as an Aura.

The last ability applies to creature cards in any zone, provided something is allowing you to cast them. For example, if you control a Zombie, you could cast Gravecrawler (a creature with “You may cast Gravecrawler from your graveyard as long as you control a Zombie”) from your graveyard as though it had flash.