
I agree with all of you theories except one. In the season finale, glasses Mugo realized that someone else had taken his appearance and told Jaheon exactly how to survive and it is def the 4th Mugo. Because glasses Mugo wants the sacrifices to continue. That's why he was so hellbent on sending Jaheon back.

That was chapter 1 I think. Even on that chapter, the way Glasses Mugo treated Jaheon was different. When he said something along the line like "It's you." at the very end of chapter 1, many interpreted that as Glasses Mugo recognizing Jaheon as the child he chose during the selection but after many chapters, I came to realize it might not be the case...he was saying "it's you" since he recognized Jaheon...from the past.

Oh, yes! Thanks for reminding me of this. So the first chapter "Mugo" might have indeed be not any of the 3 Mugo but the 4th one? Now, I wanna know who he is...the human and original Mugo perhaps? In chapter 1, his hand was shown gripping the back of child Jaheon's back. He was probably gambling on that very moment to save him? I have to re-read all chapters again coz I probably missed scenes where the 4th Mugo was shown.
They're quick in noticing the 4 "No's". I remember that scene from a past chapter where they showed the 3 Lord Mugo personalities in a staircase. At the very buttom, you see a black figure with glowing eyes. My theory here is when 3 of the Mugo personalities unite...it's giving birth to the 4th and PROBABLY the real Lord Mugo.
I was thinking the original Mugo was human too. But he was sacrificed in that jar. From what I can get from the MC's dream, Jaheon, in his past life, was very close to Mugo. However, Jaheon's his family used him to get Mugo to this jar and became a sacrifice. The person being unalived when Mugo was outside was the past was probably Jaheon too. That was way Mugo was so filled with hatred towards the head family. But since he is bounded to them due to this curse that turned him into a God, he decided to eliminate the whole lineage until none was left. But as time goes by, his conscience due to these unnecessary killings also pushed him to split into 3 different personalities. One that holds wisdom (Glasses Mugo), one that holds pure desires (Mugo the Pervert) and one that holds hunger (Childish Mugo).
I guess it also explains why Glasses Mugo was specifically gentle with Jayeon when he was a child. He was also protective of him. He gave Jaheon his blood to protect him from being harmed since he might be the only one who immediately recognized him as the reincarnation of the past "Jaheon".
These are just theories of mine based on the details from past chapters. Please take it with a grain of salt.
P.S. Regardless if this is theory is near or far from what the future plot unfolds, one thing is for sure. I want ALL of Jaheon's head family memberd DESTROYED, not just k-worded but freakin destroyed, that they will see and feel never-ending fear and pain, hopefully even in the afterlife, too. Please make it happen Lord Mugo.