Q: which scenes do you think best show how much these two protagonists (Slaine, Inaho) have changed?
I would have to say episode 12, without a doubt. For the Slaine we met at the start of the story, his reaction in this scene could believeably have been to run to Asseylum and compete with Inaho to protect her. That he didn't shows that, due to his understanding of Saazbaum's great cause, his view of the world in which Princess Asseylum was held above everything else has been shaken greatly.
Another thing about that scene is that Inaho's actions after Asseylum is shot are completely different from what we (the production committee)* originally envisioned. That scene has changed a lot from what we planned at first. Originally, even though Slaine turned his gun on Saazbaum, he was meant to be unable to shoot because he had started to sympathise with that great cause. On the other hand, Inaho didn't turn towards Asseylum, but instead tried to finish off Saazbaum by shooting him, even though he himself was heavily wounded. However, because he was reeling (from his injuries), he would miss several times, and Slaine would shoot him in the end.
So the end result of Slaine shooting Inaho remained the same, but the way it happened was completely different.
At the start, we thought that Inaho wouldn't really change even though he was acting alongside Asseylum. But as the story progressed, Asseylum's existence had much more of an impact on him then expected. Until that point, Inaho had always strived to complete the objective, based on that cool, logical side of his. The fact that he has done something so unthinkable, given his personality, shows just how much he's changed.