You don't choose which creature card exiled with Valki that Valki will become a copy of until that ability is resolving. (In many cases, the value you chose for X will give away your intentions.)

In the Commander variant, a double-faced card's color identity is determined by the mana costs and mana symbols in the rules text of both faces combined. If either face has a color indicator or basic land type, those are also considered.

If an effect allows you to play a specific modal double-faced card, you may cast it as a spell or play it as a land, as determined by which face you choose to play. If an effect allows you to cast (rather than "play") a specific modal double-faced card, you can't play it as a land.

To determine whether it is legal to play a modal double-faced card, consider only the characteristics of the face you're playing and ignore the other face's characteristics.

While resolving Tibalt's last ability, you'll add even if you don't exile any cards.

The emblem given to you by Tibalt allows you to play cards exiled with that specific Tibalt, Cosmic Impostor, even after that Tibalt leaves the battlefield. If a different Tibalt, Cosmic Impostor comes under your control, it's a new object (even if it's represented by the same card). Of course, the new Tibalt will also give you an emblem so you can play the cards he exiles.

The cards exiled by Tibalt's loyalty abilities are all exiled face up.

There is a single triangle icon in the top left corner of the front face. There is a double triangle icon in the top left corner of the back face.

Unless an effect allows you to play additional lands that turn, you can play land cards exiled with Tibalt only if you haven't played a land yet that turn.

Playing the cards exiled with Tibalt follows the normal rules for playing those cards. You must pay their costs, if any, and you must follow all applicable timing rules. For example, if one of the cards is a sorcery card, you can cast that card by paying its mana cost only during your main phase while the stack is empty.

A modal double-faced card can't be transformed or be put onto the battlefield transformed. Ignore any instruction to transform a modal double-faced card or to put one onto the battlefield transformed.

If an effect allows you to play a land or cast a spell from among a group of cards, you may play or cast a modal double-faced card with any face that fits the criteria of that effect.

If an effect begins to apply to Valki before it becomes a copy, that effect will continue to apply.

The mana value of a modal double-faced card is based on the characteristics of the face that's being considered. On the stack and battlefield, consider whichever face is up. In all other zones, consider only the front face. This is different than how the mana value of a transforming double-faced card is determined.

You can activate Valki's ability multiple times in response to one another. This may briefly allow Valki to copy different creature cards. You'll get priority to cast spells or activate abilities in between each of Valki's activated abilities.

If there are no creature cards exiled with Valki with mana value equal to the value of X as Valki's activated ability resolves, nothing happens. God of Lies, indeed.

If another object becomes a copy of Valki, it will become whatever Valki is copying. That object remains a copy even if Valki leaves the battlefield.

If Valki leaves the battlefield before its enters-the-battlefield ability resolves, each opponent will reveal their hand, but no cards will be exiled.

If an effect instructs a player to choose a card name, the name of either face may be chosen. If that effect or a linked ability refers to a spell with the chosen name being cast and/or a land with the chosen name being played, it considers only the chosen name, not the other face's name.

Valki copies the printed values of the exiled creature card. Notably, once Valki becomes a copy of another creature card, he won't have his own printed activated ability.

If Valki becomes a copy of a creature during the same turn Valki enters the battlefield, you can't attack with him or use any abilities he gains.

If an effect puts a double-faced card onto the battlefield, it enters with its front face up. If that front face can't be put onto the battlefield, it doesn't enter the battlefield.