Mutated Cultist

Devoid works in all zones, not just on the battlefield.

Devoid doesn't affect the color identity of the card for the purposes of the Commander variant. For example, while Abstruse Appropriation is colorless because it has devoid, its color identity is still white and black, and it can't be included in a Commander deck where the commander's color identity doesn't include both white and black.

A card with devoid is just colorless. It's not colorless and the colors of mana in its mana cost.

Mutated Cultist's triggered ability will resolve before Mutated Cultist does. If Mutated Cultist is countered or otherwise leaves the stack in response to that triggered ability, the triggered ability will still resolve as normal.

To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions (such as that of Mutated Cultist's second ability). The mana value of the spell remains unchanged, no matter what the total cost to cast it was.