I know that more powerful entities than Tyrion helped us in the battle against the others villains. I'm just pointing out that those villains could be held/weakened by us long enough for such entities to act. Heck, according to Chronicles Lei Shen had the power of A TITAN, and he fell to us without any help. The Lich King proved to be capable to instatly wipe the same heroes + Tyrion with little to no effort, to the point that he was playing with us all the expansion.
Again, this is just bad writing from Blizzard, but it happened and it's canon, and has to be taken into account when comparing the various villains of the series.
KJ did not make either the Plate of The Damned nor Frostmourne, the nathrezim did. They were supposed to be the "husk" for Ner'zhul soul, not the source of the power. Ner'zhul actually developed most of it's power on his own once freed from the rules of the living, without his jailers noticing. This is well explained in the books during The War of the Spider.
Despite the power creep of the Lich King figure in general (Ner'zhul - Arthas - Bolvar), I doubt it can come close to KJ, but it would be equally foolish to think that KJ can easily stops or revert something that got out of his hands in the first place.
Or not.
Visions are per se insiginificant, if they don't come to pass. Velen also saw Anduin at the head of the army of light defeating the legion, while apparently NOW the savior of all will be some kind of holy Illidan.
This is just again bad writing from Blizz side, we can take it into account, but until we actually see with our own eyes (alternate universe similar to End of Time) the scourge being toyed with by the Legion, velen Vision has no relevance.