Great assessment!
If I may add one thing to your statement of saying that the arc of the two characters is built around how they contrast one another, I would say that is true, however, they're also not so different. In BvS, Batman has already gone off the deep end, so to speak. He already no longer has faith in mankind and is a cynic as far as believing in the hope for it. He doesn't think good exists anymore because he once stood for it, but instead of him changing the world, the corrupt world just pushed back twice as hard. Superman represents the man Bruce Wayne started out as, just as Clark is now starting out on his endeavor to do what he can to help with what he has. However, he's finding out that everything isn't unicorns and rainbows, per se. Very prominent in the scene after the Capitol building bombing, with Superman overlooking the DC skyline from Lois's hotel room balcony. One of the most poignant scenes in the film, in my opinion, as he's realizing that everything Jonathan warned him about in MoS is coming to fruition. The question is, what path Superman is going to take? Is he going to let the cynical world change him, the way it changed Batman, or is he going to persevere? Superman chooses the latter and as you mentioned, in order for Batman to be brought back, he had to first be brought to the darkest depths, which was killing in cold blood with no contrition.
The one thing that I personally thought would have been a nice little addition in the famous "Martha" scene, was when after Superman said "Martha" and Batman had his flashback of the murder of his parents, it should have showed the face of the gunman after the gunman shot his mother, after showing the bullet casing discharge, but instead of it being the face of the actual gunman, it be Bruce's face. Thus, further illustrating to the audience that he was realizing he was becoming the same man that murdered his parents in cold blood in that dark alley. Would have also been an extremely poignant scene in my opinion, especially with the score that was playing in the background.