With
Sneak Attack, the Imposter-whatever will definitely have haste. Sneak Attack gives whatever it's putting onto the battlefield haste, regardless of whether or not it's a creature, and noncreature objects can have haste just fine, even if it doesn't actually accomplish much of anything.
But for
Grave Betrayal, whether or not it's a Zombie depends on the cards involved and how it's being animated, because noncreature objects can't have creature types. If due to timestamps Grave Betrayal's effect tries to turn the Imposter into a Zombie before it becomes a creature, the effect fails, and even if a later-timestamp effect animates it, it still won't be a Zombie.
In general, if the animating effect predates the Betrayal trigger's resolution (such as a
March of the Machines already on the battlefield), it
will be a Zombie, because it becomes a creature before the Betrayal tries to zombify it. If the animating effect is created some time after the Impersonator enters the battlefield, it will not be a Zombie, because the animating effect has a later timestamp than the Betrayal's effect.
If you'd like to play around with edge cases, I suggest heading to
Rules Theory and Templating; those folks will be happy to discuss it with you and walk through all the various possibilities.