For your first question, Slaking will not be able to move on the second turn since Truant alternates. Lets say Slaking was sleeping on the first turn, but no one woke it up that same turn. The ability Truant doesn't really effect the way Slaking moves since there's a chance of it not waking up anyways, and even if it does, it won't be able to move because of Truant. Therefore both cases will result in it not moving the second turn.
For Instruct, it acts as a separate condition that turn. For example, lets say your doing 2v2, starting off with Slaking and Oranguru. Slaking and Oranguru both attack the first turn. Since slaking is faster, when the second turn comes around Slaking will loaf around for his turn, but when Oranguru follows up with Instruct, Slaking will use its previous move. With this strategy, Slaking will be able to attack every turn, but will need to use the same move. But, if Slaking moves after Oranguru's Instruct, Instruct will trigger the Truant ability, then Slaking can move afterwards. This strategy allows Slaking to attack every turn, and use any move in it's move slot.
For Uproar, the move will stay in play, unless the move fails by not being able to execute. Therefore, Slaking with Uproar on its own will execute it the first turn, but have it fail on the second turn, therefore only allowing Slaking to attack every other turn. But, if you use the Slaking/Oranguru Instruct strategy, you can use Uproar every turn.
For encore, Slaking will attack according to Truant.
I've tested these on Pokemon Showdown, and it does indeed work according to your rules
Hope I helped! :)