Check to see if there are two or more instant and/or sorcery cards in your graveyard as the spell resolves to determine whether the spell mastery ability applies. The spell itself won’t count because it’s still on the stack as you make this check.

You draw a card as Dark Dabbling resolves, not as the creature actually regenerates.

Dark Dabbling is still on the stack while you count the number of instant and/or sorcery cards in your graveyard. It doesn’t count itself.

If the target creature is an illegal target by the time Dark Dabbling tries to resolve, the ability doesn’t resolve. You don’t draw a card and no creatures are regenerated.

Dark Dabbling’s spell mastery ability is a normal spell effect, not a replacement effect. This means that if you replace drawing a card for Dark Dabbling’s effect with dredging a card from your graveyard, the cards put into your graveyard this way may enable the additional regeneration.