2019-01-25
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.

2019-01-25
If Skarrgan Hellkite's ability has two targets and one becomes illegal, the remaining target is dealt 1 damage, not 2.

2019-01-25
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.

2019-01-25
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.

2019-01-25
If a creature entering the battlefield has riot but can't have a +1/+1 counter put onto it, it gains haste.

2019-01-25
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.