Addendum abilities of instant spells apply while the spell is resolving, not immediately after casting it. If the spell is countered, you don't get the addendum bonus.
2024-01-12 • wotc
If an effect copies a spell with an addendum ability while it's on the stack, the copy wasn't cast at all, so you won't get the addendum bonus.
2024-01-12 • wotc
If a card in your hand has {X} in its mana cost, X is considered to be 0.