"That spell's mana value" refers to the spell that caused the ability to trigger, not the card you look at when you scry.

If the instant or sorcery spell has {X} in its mana cost, X is the value chosen for X as the spell was cast.

The wrong card name was inadvertently included in Zaffai's printed rules text. The text above is correct and the full name of the card is Zaffai, Thunder Conductor.

Each magecraft ability has a different effect, although they all have the same trigger condition, whenever you cast or copy an instant or sorcery spell.

For example, if you control Archmage Emeritus and cast an instant or sorcery spell, Archmage Emeritus's magecraft ability will trigger and you will draw a card.

If an effect creates a copy of an instant or sorcery spell, this will also cause the magecraft ability to trigger.

If an effect creates multiple copies of an instant or sorcery spell, magecraft abilities trigger once for each copy created by the effect.

Some effects instruct you to copy an instant or sorcery card in a zone other than the stack. These copies do not cause magecraft abilities to trigger. However, most effects that do this also allow you to cast the copy, and casting the copy will cause magecraft abilities to trigger.