QIf I have a Shadow's Corrosion in my cemetery, but there are no valid targets for it, how should I resolve option 2 of this follower's On Evolve ability?

AFirst, select the Shadow's Corrosion in your cemetery. Then, because there are no valid targets for it, you won't be able to play it, so nothing will happen, and it will remain in your cemetery.

QAfter I resolve option 2 of this follower's On Evolve ability, do I put the Shadow's Corrosion back into my cemetery?

ANo. Since you have a Valdain, Cursed Shadow on the field, you will put the Shadow's Corrosion into your EX area with its ability.

QWhen playing this follower's On Evolve ability, if I can't select an enemy follower on the field, can I still choose option 1 to search my deck for a Shadow's Corrosion?

ANo. If you can't select an enemy follower, you would be required to choose option 2. Likewise, if you can't select a Shadow's Corrosion in your cemetery, you would be required to choose option 1. However, if you can't select a suitable target for either option, then you can't play this follower's On Evolve ability and nothing would happen.