How Is God Merciful ?
Please do not think about it from a human perspective. The ability of mercy was always there. The same way the ability to speak the Quran was always there.
As I already stated, unless there exists someone to show mercy to, one can’t be merciful. Ability to be merciful later is a vague concept. That’s why I say that writers of Qur’an (who were humans) didn’t think through these concepts properly.