I 100% agree with you that they messed up Frostmourne. Originally it was supposed to enslave a soul to the will of the Burning Legion, but that somehow got retconned to being a psuedo-servant to the final entity of what the Lich King was supposed to -become-, to just being enslaved to the sword itself and all kinds of mess.
I always figured that Ner'Zhul was never loyal to the Burning Legion, and was originally supposed to merge with Arthas in order to, as you said, simply be free to be mobile. That thought changed when instead of ordering Arthas to return immediately to him after destroying Lorderon, he instead sent Arthas to be the Legion's temporary errand-boy in helping Big K reach Azeroth.
I think it again goes back to having problems modifying lore to suit the MMO frame. They wanted to hype up TBC, as that was going to be their then-current expansion, not WotLK. Why not have arthas run a few errands for the Legion, set up a lovely forsaken plothole, then retreat to icecrown and merge with the Lich King to become this... hodgepodge mass of personalities and ambitions.
Perhaps all that time the Lich King was sitting up in Icecrown after the merge was him sorting out the minds and souls within that body. I don't know. I'd like to see them release a series of books that would simply tell the straight story, without interference from either the Warcraft games OR World of Warcraft. I really enjoy the plot, but throwing us adventurers in the way and making things revolve around us just makes it too hard to separate fact from fiction and retcon from original idea.
Just look at what they did to everyone's favorite Space Goats.
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