
Anyone know what Hyunsung is hiding? Spoilers are very appreciated.

By what specifically?
He's a regressor but only Kiyoung knows that (Hyunsung didn't know that Kiyoung knows). If you're talking about why he asked Kiyoung for help to not make Charlotte the Empress, it's because they've been close friends in the first life. He saw how Charlotte became devastated after she became the empress.

I think Huimin is actually crazy and really does have DID. That's why the ML wanted the original paintings gone and was concerned about MC remembering. The paintings were probably a trigger. The ML is def a regressor that probably got tagged in after the MC's heat. The reason I'm still going down the MC might actually have DID route is also cause he muscle memoried the painting he made and still manages to recall things he shouldn't. A lot of reincarnation manhwa/novels, when they have this plot point, tend to end on some kind of plot twist like that.
The MC lived in the OG time line at one point with the original ML then died. Then cause the ML keeps regressing the MC is also retaining the original "memories" as a book because his trauma will not let him believe he actually lived a life like that. The alt reality probably also happened but as a fantasy, or a fake reality, the alter MC used to cope and told a previous ML. He remembers these thing then is trying to help the MC live through his desires to keep him happy and mentally healthy.

The princess made a dumb af move with what she just did. Doesn't matter how much support she has from the nobles. The church, and the other transmigrators would take his side. He's also the leading supplier of potions as well as being the guy who just invented TV. The propaganda this guy could push out is crazy. She's absolutely cooked.
They look like Aventurine and Dan Heng (Imbibitor Lunae vers.) I don't think it was intentional but if anyone ships this rare pair ig they have options now.