Since ancient times, a colony of mold referred to as "mycorrhiza" (菌根) has decomposed animal and plant tissues using haustorium-like organs. When Eva passed away and was buried, the mold reached her body through the soil and decomposed it, transferring genes and consciousness, which were eventually dissolved within the center structure. So Miranda's plan was plain and simple: she needed to find an adapter to transfer her daughter's consciousness into this body via a ritual so that it would regenerate.
The first steps of Miranda's research were parenteral infection. She used a basic syringe to inoculate people with mold. Some people's bodies rejected this fungus and died, while others adapted but turned into senseless monsters. Sure, Miranda could have used any of these mutants as a "vessel", but it would have been an unefficient "vessel", so the progress she made by infecting several hundred people wasn't satisfactory.
Trying to figure out her next steps, Miranda came up with the idea of creating a biological vector that could increase efficiency in question. She used a roundworm and, by integrating the mold genome into it, engineered an organism that can adhere to human nervous tissues and produce mold to cause infection. Most people also either died or turned into monsters, but Miranda managed to create semi-compatible cases that became lords.
Trying to go further, she started cloning her daughter. This is evident in the Japanese version of Miranda's diary, where she mentions that she gave The Connections Eva's embryo, not her DNA. Although the game doesn't explain how Miranda created the embryo, it's not hard to imagine that she found an oocyte donor in the village and replaced its nucleus with another one with Eva's genetic material from mycorrhiza. Miranda could have tried to create a biological vector with a human embryo instead of a roundworm, but reached a dead end because developing a bunch of cells into a complex functioning body is hard.
But The Connections found her and offered to assist in the research. Together they began to develop biological vectors by trial and error. Before Eveline was created, there were many girls with bodily deformities and other developmental issues, but even Evie had congenital chromosomal defects that caused her to age 25 times faster than usual. This was ultimately the reason why Miranda decided that the girl wasn't a suitable "vessel".
The mold that Eveline produced is 0.05% genetically different from the Eastern European mold. This mycelium is an extension of the girl herself and stored her genes and consciousness. And only this particular type can create Molded, most likely due to Evie's genetic defects. Instead of gradually replacing the human cellular structure, the fungus does a rush and harsh job, making a human merely a shadow of their former self.
The fact that the Cadou parasite and Eveline were created through the integration of the mold genome is evident by their designs as well:
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The Romanian word
cadou means "gift", and Eveline gave gifts to people by infecting them, so these vectors are connected by a common idea at various levels.