Kira grants an ability to your creatures that triggers when the creature becomes targeted. But if Mutavault is targeted before it actually becomes a creature, it will be too late for the ability to do anything to a spell or ability already on the stack.
If Mutavault is a creature when it dies, it will trigger Athreos and will be returned to your hand unless the opponent pays 3 life.
Drown in Sorrow only affects things that are creatures at the moment it resolves. Any creatures that appear later in the turn won't be affected!
is back in town, and doing fun things! It swings for 2, it gets pumped by , it has the subtypes of Ninja Mutant Turtle- and it taps for mana! What a land. However, some people don't know how to properly attack with it! You see, to swing with your Mutavault, it has to be a creature when you start declaring attackers, which is the absolute first part of the Declare Attackers step. So, by-the-book, it needs to be a creature BEFORE that point. Your last chance to animate it so it can attack is during