
Like, why Regina has healing powers when the original one didn’t, why Arsene is the antagonist in the original when the Empress Dowager is said to be evil… also, why Isotta fell in love with Ruari when he’s the one who ordered Arsene to kill Regina. Why Arsene had to kill himself to “save” Ruari?? It doesn’t make any sense.

Can you imagine if the OG had healing powers? Arsene would have killed her over and over in different ways till he found one that sticks...
I think the empress dowager might have been the act 1 villain, where Isotta falls in love with Arsene during her investigation into Regina's death.
Then, when empress is pushed back somewhat (possibly not defeated yet), Isotta meets Ruari. Due to ML powers or something, she likes Ruari better, causing Arsene to do something stupid?
Maybe at this point, either the empress or Ruari take the opportunity to tell Isotta how Arsene was really the one who killed her friend without going into too much detail about the why or how. If empress says this before she's rescued by Arsene, now she resents and doubts him. If Ruari either says this or supports the statement after Arsene betrays him, he can drive Isotta to distrust Arsene further.
After this, I suspect something happens that hurts Isotta, maybe? Possibly due to Arsene's actions? Either that or empress gets something on him/using him to move against Ruari, and by extension Isotta. If magic is a thing, maybe some sort of control magic? He kills himself to stop her plans through him
But hey, that's just a theory
There are a lot of plot holes in this story. But I’ll ignore them for now.