A retrace card cast from your graveyard follows the normal timing rules for its card type.

The subtype Shapeshifter that appears on the type line is mostly there to reinforce the flavor. A spell or token with changeling is just as much an Elf, a Dwarf, a Sliver, a Goat, a Coward, and a Zombie as it is a Shapeshifter.

Casting a card by using its retrace ability works just like casting any other spell, with two exceptions: You're casting it from your graveyard rather than your hand, and you must discard a land card in addition to any other costs.

If the active player casts a spell that has retrace, that player may cast that card again after it resolves, before another player can remove the card from the graveyard. The active player has priority after the spell resolves, so they can immediately cast a new spell. Since casting a card with retrace from the graveyard moves that card onto the stack, no one else would have the chance to affect it while it's still in the graveyard.

Changeling is a characteristic-defining ability. It functions in all zones.

When a retrace spell you cast from your graveyard resolves, fails to resolve, or is countered, it's put back into your graveyard. You may use the retrace ability to cast it again.

The value of X is determined only once, as Formless Genesis resolves. The token's power and toughness won't change if the number of land cards in your graveyard later changes.