You're choosing sliver, if you weren't sure
I am tempted to try Magic Assistant at some point in the future, but the sheer volume of cards means that putting everything into the system will take a long time
If you cast them during night, they will transform right as they leave the stack and enter play already night-side-up
Like most Pokmon cards, the value of Mimikyu depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Lost Origin)
Even though the humble Negate may seem like a better exchange of mana-to-possible-counter, remember that opponents are often tapping out to cast their huge Fireball and not expecting to see a Spell Pierce countering their spell as well as a Counterspell would, anyway