If the colorless spell has in its mana cost, use the value chosen for X when calculating that spell's mana value.

Skittering Cicada's second ability applies only to casting spells. It does not, for example, change when you may activate abilities that can be activated "only as a sorcery."

Skittering Cicada's last ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered.