In light of the newest update, I have a revised theory:
What if Yingxing was infected with Shuhu's cells before the Sedition?
There was a huge chaotic battle with this thing that likely had his flesh and blood (bark and sap?) flying everywhere and landing on people. (I imagine a huge draconic Dan Feng just absolutely ripping into this big tree monster). The Xianzhou weren't affected because they are *technically* already Abominations of Abundance (infected by the Ambrosial Arbor). But a plain short life human could absolutely be affected by even the tiniest cells. The Happiness Society members were infected by a microscopic collection of Shuhu's cells, so who's to say Yingxing couldn't be infected in the same way?
Let's say Yingxing got covered in Shuhu's blood and guts during the battle. It seeped into his body and lay dormant there until some sort of trigger. Ashveil was the one who eventually killed Shuhu and ended the battle, swallowing it up completely with his shadow, even the parts that scattered across the battlefield. But what if the shadow missed a small part attached to Yingxing? Either because Ashveil sees him as an ally, or he simply couldn't detect such a faint scent of Abundance on him (especially surrounded by other Xianzhou folks who also have dormant Abundance cells).
So now Yingxing is infected with Shuhu's flesh, but he's not showing any symptoms of Abundance or mara until something triggers it. Him and Dan Feng clear the palace ruins at Scalegorge for whatever experiment/ritual they were doing, and something during that experiment triggered it. And THAT'S why Jingliu killed him. Yes, Jingliu was on mara-watch, but she didn't have it *yet,* and I can't imagine her killing one of her close friends for no reason. Yingxing suddenly becoming mara struck out of nowhere would absolutely be a justifiable reason for her to freak out and kill him in front of Dan Feng.
On the subject of Dan Feng, I can't imagine him purposefully infecting Yingxing either though that is what I was initially led to believe. (And honestly it's still a possibility. Dan F/Heng will go to extremes to protect his companions, he even admits it himself). It's likely whatever ritual they performed triggered Shuhu's dormant cells in Yingxing, like how the Happiness Society members had dormant cells until triggered and they transformed into beasts (hmm). This would leave Dan Feng totally blind sided and *devastated*... enough to allow himself to be arrested (cause come on, there's NO way he would've gone to the Shackling Prison if he wasn't willing OR extremely injured, which is another possibility. Dan Feng is ridiculously strong and could probably wipe out a Xianzhou ship if he *really* wanted, especially surrounded by his ancestral waters). While I do love the narrative of a slightly evil/morally grey Dan Feng who would use even the flesh of his enemy to make his lover immortal, I'm more inclined to believe he's smart enough to understand that is a VERY bad idea, even as arrogant as he is. He would've had to preplan and consciously make a decision to sneak into the Shackling Prison and take Shuhu's remains (unless he was given the remains by Sanctus Medicus, which is also a possibility given it's implied Dan Feng was tricked by them).
(Did the Xianzhou even have Shuhu's remains if Ashveil swallowed him completely? No one’s even allowed to check. Hm Xianzhou propaganda makes lore sleuthing very difficult lol).
In short Yingxing being unknowingly infected by Shuhu from that battle would explain Jingliu and Dan Feng's behaviors around the Sedition (what little we know of it).
Side tangent: How does Blade have such good control over mara/Shuhu? Imagine this, Yingxing is infected by dormant cells. Whatever ritual him and Dan Feng perform triggers it, turning Yingxing into a mara struck beast. Dan Feng panics and uses his dragon heart to stabilize him (either before or after Jingliu comes over and kills YX).
That's right. Dragon Heart theory. Jazz hands.
It would also explain why Dan Feng allows himself to be arrested. He *severely injured* himself to stabilize Yingxing, and likely drained a lot of his power/energy to make it work. It explains why he doesn't have his dragon heart in the Prison. It explains why Blade isn't visually affected by his immortality/mara other than his eyes (which look shockingly draconic). It explains why Blade feels calmer being with Dan Heng (you can hear heartbeats when he looks at him). He's essentially with his other half. It explains why Motenax Blade's splashart is in the shape of a heart.
(I've been a long time believer of the dragon heart theory. I came upon the conclusion myself when I first played through, but it's only recently I discovered it's actually a popular theory).
Now whether the "dragon heart" is metaphorical or literal is to be decided. Personally I'm for it being literal because I enjoy body horror (especially from the Xianzhou those freaks have body horror everywhere). Dan Feng is also a biologist/doctor, so it's something he could absolutely attempt to pull off (we talk about his cool magic dragon powers a lot but let's never forget he's also an old fart smarty pants scientist). Something about giving your *literal* heart to your lover so that you can find each other in your next lives is so tragic and beautiful and fits their theming.
Mortenax Blade has a suspiciously placed marking on his chest that seems significant, which could point to the dragon heart. The phoenix motifs also align with Vidyhadara themes of rebirth (and connects him to Dan F/Heng, since phoenixes and dragons are seen together as a symbol of unity). The fire element could point to draconic features, but it could also just be a reference to being the "Furnace Master." Overall Blade is definitely going to gain some sort of enlightenment/insight/control over his body, so this would be a perfect time to reveal something about the dragon heart.
TLDR: Yingxing was infected by Shuhu before the Sedition. Dan Feng's only interference with his mortality was giving him the dragon heart to stabilize him.
Getting my scattered thoughts out before 4.3, cause the next update will almost definitely give us more details about Yingxing, Shuhu, and the Sedition.