I dmed Kenshirou Morii to ask about the ED.

He said:
FA監督の入江さんに「森井の絵柄でアニメを作って欲しい」と言われて、鋼の錬金術師の世界観とどうマッチさせようかと考えた結果、アルとエドの描く落書き風と言う表現を思いつきました

He added later that the cuts were split among all 3 of them, but he doesn't remember who was in charge of which section since it's been so long ago. But chances are he was in charge of the most ambitious cuts since he's been drawing in that artstyle for a long time.