What's Yours Is Now Mine is not legal in any format.
Rulings
2017-06-13
Auras and Equipment attached to that creature remain attached, but you don't gain control of them.
2017-06-13
This scheme doesn't grant haste to the target creature. It won't be able to attack during this turn unless it has haste or somehow gains haste.
2017-06-13
You can target and gain control of an untapped creature this way.
2017-06-13
You control that creature indefinitely. If the owner of the target creature leaves the game, that creature leaves the game as well.