Surprise! You're like a mouse so easily set and lured by a cheese in a mouse trap.
It's not obvious for a guy like you since proofs you needed have to be obvious or have to be screaming in front of you.
Maybe you're vision is a bit blurry so let me aid you in having a clearer picture.
=Illidan the protege of the Demon Hunter.
Sargeras chose Illidan a sorceror during Azshara's time pretty much adept with arcane. Sargeras contacted him gifted him with the gift of Demon Sight. Sargeras the brain behind the Burning Legion would definetly oversee and have his vision all over his commanders, their operations, even the minions they command.
An avatar for all time
= Avatar of Sargeras > possessed Magna Aegwin a crypto-Avatar within her. Medhivh the personification of Sargeras's plan of bringing the Orcs into Azeroth; Orc who consumed demon blood and are fel infused.
Demonic visage
=Illidan the perfect replica of Sageras. horns, hooves,wings, claws, burning eyes. The only thing missing is a sword similar to gorshach aka new frostmourne and Bolvar's charred body and do fusion.
I'll be back and make a comment specially for you Marcellus