Almost nothing has been revealed, so the truth is really unknown at this point. I doubt everything is clear cut from a few hazy memories that the FL has had and the weird accounts from the maids. But I will say this: I don’t think the original owner of the FL’s body was actually Noevan’s(?) mistress. The OG could have blackmailed him into doing things for her based on the note the now FL received from him. That or they made a deal to get something important done in secret, and they were using the “mistress” cover to keep things under wraps.
I have read much worse translations and this really isn’t bad at all. It’s actually quite good compared to some of the things I read on here whenever they update. But as for the name change, I don’t know if it actually changed in the translation or if someone in the story marked her name down wrong on the reply for the funeral attendance. Because as far as I know, the FL’s nanny/head maid(?) is named Maji, and the men looking for FL think she’s a maid of the count because of the name. So there is some confusion there for sure.
Ignoring the translation issues (for you and others), it’s reasonable to feel confusion overall at this stage of the story since it just barely got started. If the author/artist reveals everything right away, it’s just not interesting. You have to hint at things and keep people guessing. And that is what has been done here, in my opinion.
The FL (Adrien) died and how now become Blie (black haired), who is assumed to be her husband's mistress-which, by the way, I think is true? Then, there is the crown prince who might harbor some feelings for Adrien. He went to war and come back with the news of Adrien's death, hence why he suddenly rushes out to her grave and cried. I got the feeling that the blonde prince is the ML but I don't know. The story is confusing, probably because of the translation? But yeah, that's the gist of what I think I understood from reading it.