The await is on the withCheckedContinuation call, so execution can suspend right before that call, it's not guaranteed that withCheckedContinuation will execute after the previous statement without any suspension in between.
I don't quite get what you mean. Are you concerned that putting additional sleep calls will somehow allow simultaneous access to the actor state from multiple places? That shouldn't be possible, no. When a task is suspended, it is inert, so it is not accessing an actor's state. Before a new task can execute on an actor, the running task needs to suspend or finish.