She's an honorary duchess because of her position as a member of the royal family, but the title comes with no influence or lands, probably just a monetary allowance. The marchioness title will entitle her to all the land, resources and money of this family, plus it would make it it hereditary for her children, and it would not interact with her succession rights. Its not a downgrade, its an adjacent title. Its pretty common for member of royal houses ro.obtain secondary titles this way.
She’s aiming to be empress so she can give homunculi rights instead of slaves, so why would she ever accept it?
I get it’s so she can have a high ranking family backing her via as her husband, but she’s gonna bag the king of homunculi.
from duchess to marchioness? yea aint noone want that title drop