Who somehow managed to stumble upon a cave that so happens to have the strongest mold colony on the planet, that is far fetched and unbelievable to me even for a science fiction franchise.
These lands had a rich history with this fungal root.
At the time of Romanization, its power was used by the so-called "Demon King", who led the Roman tribe.
In the 15th century, he was overthrown by a Romance group from Western Europe, who established the Chalice to control the growth of mold.
This root has been acknowledged by many people, and many have shed blood for it. We can even see the crystal remains under the stronghold. And there are plenty of such remains, as we find minerals in all locations. Thanks to this, the village even fond of mining.
Miranda was just a local villager who wasn't the first to have this contact.
Spenser had to do extensive research and use up countless resources to even find and retrieve the Progenitor Flower’s, and even then they weren’t producing the virus because it was removed from its natural habitat.
Sure, but this is the path he chose himself. He wanted to evolve and become an immortal God. These ambitions were beyond his competence, although, to be fair, he had every opportunity to achieve this. If he had at least been more patient, he could have saved some Weskers.
You would think Spenser would of gave the Megamycete a second look instead of spending his entire fortune and life on a what-if virus that was not guaranteed results.
That's why we have Spencer's right-hand man, with whom he worked in Africa, and his organization to begin with.
Spencer has no reason to associate himself with mold, and neither does Umbrella. This is contrary to their doctrine. Viruses are the engines of evolution, and they use these agents to create superhumans. Mold, on the contrary, is just "Marcus' leeches" at the intracellular level. Mold simply steals your genes and condemns itself to mortality, because our bodies are programmed to die.
Meanwhile, after meeting Miranda chronologically, Spencer tried to create the ceremony technology with the transfer of consciousness, but he abandoned it. Probably because he came to the realization that the actual transfer is impossible or at least unavailable now. This is even more reason why you don't want to accept mold, since this is the pure Theseus paradox.
Unless I am missing something but she didn’t seem like the world conquering type before becoming immortal. Like you said, she lost her humanity after merging with the mold for so long, so what was Spenser’s excuse for acting out such atrocities against humanity?
As they say, power corrupts. Absolute power corrupts absolutely. Once Spencer could have been a nobleman with pure intentions, but this fascism grew in him when he realized that he could be superior. And many things, be it his name and the crest in the cave, which is similar to his family's, were like signs of fate that he was chosen to carry out such a mission.