I really liked the second couple but man this historical arc is so long and I don't unders...

That's what's anazing about this story, my friend. There's no new characters (technically), because all of them are just the original characters ancestors. To summarize:
-The pale skin man with long black hair that died is just 704 before becoming a reaper.
-The blue hair is just our lovely goblin Geumsun.
-The white haired man with the bow is San's ancestor (that basically built the ground for the whole story to unfold)
-And the pale man with his hair up, who got possessed by Hwan is Eunjae's ancestor (that I believe is responsible for all the misfortune Eunjae has in his current live).
This arch is indeed long, but it's also essential to the main story because now we are getting the whole picture from the plot. Befere this we only had some fragments: Hwan is a monster, he is bad, he possesses people and he needs to be killed. He wanted to possess Eunjae for years (now we understand why) but he was injured thanks to some battle he had with Geumsun (That's exactly what they are showing us in recent chapters) So Geumsun and Yama got San to help them take him down ( We didn't know why him, but now we see that his ancestor was the one who gave them the tools and that's why San is so important), but in the end it was 704 the one holding the bow to finally take Hwan down (Now we know that 704 died in his past life killed by Hwan, so that's probably the reason he was the one in the prophecy). I don't know if all this made sense, but if it did you will see how beatifully written this story is and how important this long ass arch is for us to finally finish this puzzle. It all fits perfectly

You said this so well! Story is beautifully written and honestly this arc finally helped to understand what is going on. Without it, all previous chapters woud be incomplete and shallow. I saw in the raws that season 3 is ended by chapter 110, couldn't understand a thing, but I suppose there will be season 4 as well. Somehow it didn't look like complete story, but maybe I'm wrong. Will be waiting for the author's note to translate it.

It’s not anything new. It’s telling the how the story began. I like how the author takes the time to explain the events leading up to the present and not just gloss over it. It really deepens the story and the characters. I like how it doesn’t drag on. It gives the story a background. I rather not have it continue in the present and having random plots added to make it longer.