1.KJ wasn't afraid that LK would grow too powerful to be a threat to him but KJ sent Illidan to destroy him because LK betrayed him and messed with Legion's plan. KJ couldn't enter Azeroth so all he could was sending his pawn. This is what happened in WC3.
2.Legion does have necromancy. Dreadlords are part of the legion and KJ has master all forms of arcane magic. All arcane magic you see in game could be used by him. Ner'zhul didn't pull necromancy out of his ass. He was "granted" power by KJ.
3.DW was undefeatable without Dragon Soul the most powerful artifact in Azeroth. 4 Aspects couldn't even beat him. Dragon aspects would MELT LK if they participate in the war at ICC. LK was defeatable even without Ashbringer as wa saw from Wrathgate cinematic. Just a plague bomb hurt him.
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1. KJ was stated to be better arcane user than Archimonde but there has never been official statement that he is weaker than Archimonde physically. That's what you made up.
2.LK couldn't beat Archimonde because he didn't have the world tree to sacrifice. Combined might of mortals didn't stand a chance against Archimonde. That's why Malfurion made that decision. Archimonde was destroyed by the nature herself. How Lich King would do that? Frostmourne is pathethic weapon against the like of Archimonde. It was crafted by Legion in the first place.