Responses
Spoilers: They are the same person, and honestly I hope the manhwa can do justice to the novel with how MLs obsession over the MC was definitely unhealthy and how they reconciled later on and restarted with a much healthier relationship, unfortunately I can’t quite remember where I read the novel though— I apologize!

this is just my theory, i havent read the novel and i cant find any spoilers, but what if the MLs are twins? i've read many manhwa where siblings have similar names, so yoochae could be Chae Yoo___, (like Yoohyun, Yoojun, Yoongtae, etc) brother of Chae Yoojeong.
cause the other option is just Yoojeong lying really hard and going way over the top and doing way too much, and tbh with how much the author has stressed Jihan hates liars and how many chances he's given them to come clean, idk. it just doesnt feel good.
i feel like brothers/twins seems like a valid guess
(also if anyone has a link to the novel, i would be forever grateful <3 )