
You are actually right...
Spoilers
Based on the spoilers I read, OG Fl was supposed to go to war with the ML but she was scared and a lot of things.. so since the Duke loves his daughter so much he made a ritual to transfer OG FL into the blonde hair girl and FL the transmigrated occupied the body so she was the one to go to war and such.. The Duke is nice to her and is always saying to take care of herself because he was gonna swap FL to the OGFL after the war ended and she's done her part.. the ML's were brainwashed

finally someone felt what i was thinking.. after binge reading this - that was my theory as we.. the clue for me was the chief of information guild.. he should be dead and the FL thought he will be dead since he was a traitor to the duke's house - but the "master" let him leave and gave 2nd chance.. ┑( ̄Д  ̄)┍
The dad knows FL is a fake - FL herself says her dad is "caring". The ogFL from before is a typical villainess character (being rude, violent, entitled, throwing tantrums).
Then war started and ogFL was supposed to go to battle - then our current FL transmigrates in. Complete change of character - even crown prince, the dumbass, notices. There is no way FL's father wouldn't notice.
(?maybe the dad had a play in this transmigration)
The way the Baron's house gaining all the money/income coincidentally at the same time as FL's transmigration (note, not after war victory) - from where? Possibly the duke's house. Maybe the ogFL is put into Grace's body so she can avoid going to war, and this is why the dad is supporting his OWN daughter in Grace's body by giving money to the baron household
And when he shows affection for FL, it's always "eat well, take care of ur own body" mainly cuz it's his og daughter's body.
Ok maybe im being delulu - but i think it matches the clues given so far