WotLK was hardly a bastion of good writing though, so I wouldnt consider it sacrosanct in that regard. The Frozen Throne was definitely more ambigous on whether Arthas was posessed, under subtle mind-control or whether he had actually just gone completely insane, so in that sense I think the lore of Shadowlands fits it much better.
Also, correct me if I am wrong, but wasnt the whole "Arthas was in complete control" angle something that only really existed in the tie-in novel?