Crackling Spellslinger’s second ability triggers if you cast it from any zone. It doesn’t trigger if you put Crackling Spellslinger onto the battlefield without casting it.

The copies of a spell with storm created by its storm ability are put directly onto the stack. They aren’t cast and won’t be counted by other spells with storm cast later in the turn.

A copy of a spell can be countered like any other spell, but it must be countered individually. Countering a spell with storm won’t affect the copies.

Spells cast from zones other than a player’s hand and spells that were countered or otherwise failed to resolve are counted by the storm ability.