1st question:
I control an animated Mutavault.
I cast Flickerform on it, then Darksteel Mutation.
What happens when I activate Flickerform's ability?

2nd question:
I control an animated Mutavault.
I cast Darksteel Mutation on it. Then Reality Ripple.
What happens at the beginning of my next untap step?

Answer the first: At the beginning of the next end step, the Mutavault returns to the battlefield. Then, Flickerform's ability tries to put Flickerform and Darksteel Mutation onto the battlefield attached to Mutavault, but fails because Mutavault is not currently a creature, and therefore is not a legal object for either of them to be attached to. Both Flickerform and Darksteel Mutation remain exiled indefinitely.

Answer the second: Mutavault and Darksteel Mutation phase in together, with the Mutation attached to Mutavault. Darksteel Mutation's ability is currently rendering Mutavault a creature and thus a legal object for the Mutation to be attached to, so it remains attached and on the battlefield.