2019-01-25 : 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. For example, if you cast Sphinx’s Insight during your main phase, you gain 2 life during its resolution, after you draw two cards.

2019-01-25 : If a spell with an addendum ability is copied, the copy won’t give you the addendum bonus. This is because the copy wasn’t cast at all.

2019-01-25 : If the target creature is an illegal target by the time Arrester’s Admonition tries to resolve, the spell doesn’t resolve. You don’t draw a card if you cast it during your main phase.