Paying two to copy an ability might seem a bit steep, but when you can funnel all of your mana into it late game to copy an ability two or three times, it starts feeling a lot better
(In many cases, the value you chose for X will give away your intentions.) If there are no creature cards exiled with Valki with converted mana cost equal to the value of X as Valki's activated ability resolves, nothing happens
If your lands tap for two or three mana each, Time Spiral actually nets you a massive amount of mana
On the other hand, removal that can only target nonartifact creatures, like for example Go for the Throat, can't target vehicles even when theyve become creatures they are still artifacts, too
And if you countered an ability of a dork, 'walker or artifact, they are shut down as long as you control this Merfolk Wizard, so those are permanently answered