Good on him for being so determined. And yet, I can't shake the feeling that this is either a bad move, or a cop-out. Who's to say the author lets the arms stay there and not get taken by some third-party. Apparently just anyone can go down to Excalibur. Honestly, I'll be more impressed if the arms stay there until Jimmu feels ready and gets them back. Otherwise, I'll just be disappointed for calling it out this early.
At least this does explain Jimmu's behavior and the rushed feeling of early chapters. It's all to set this up.