原文You divide the damage as you activate Skarrgan Hellkite's ability, not as it resolves. Each target must be assigned at least 1 damage. In other words, as you activate the ability, you choose whether to have Skarrgan Hellkite deal 2 damage to a single target, or deal 1 damage to each of two targets.
原文Once Skarrgan Hellkite's ability has been activated, it resolves even if all +1/+1 counters are removed from it or if Skarrgan Hellkite leaves the battlefield.
原文Riot is a replacement effect. Players can't respond to your choice of +1/+1 counter or haste, and they can't take actions while the creature is on the battlefield without one or the other.
原文If you choose for the creature to gain haste, it gains haste indefinitely. It won't lose it as the turn ends or as another player gains control of it.