Arboliva ex

If I use Arboliva ex's "Oil Salvo" attack while Granite Cave is in play and choose three Steven's Pokemon twice, how do I apply the damage?

Oil Salvo is a single attack. The attack's damage is calculated after you finish making all choices, so all damage modifiers are applied to the total damage for each Pokemon. In this example, each of the chosen Pokemon receive 40 damage (20 times two) which is then reduced by 30 for each Steven's Pokémon due to Granite Cave, so each will ultimately take 10 damage.

If I use Arboliva ex's "Oil Salvo" attack and choose my opponent's active Shaymin with the "Flower Curtain" Ability first with enough damage to KO it, can I then choose benched non-Rule Box Pokemon and do damage to them?

No, you cannot. All damage is done before KO's happen, so the Shaymin is still there to protect Benched Pokemon that don't have a Rule Box.