I think that this description is slightly inaccurate and does not fit in with what the files show us. I don't mean that this is your description. You quoted it, of course.
Miranda gave the fungus and her daughter's DNA to the organization that created Eveline, but these people made a defective organism that she refused. Eveline depended on a special treatment that supports her immortality, and Miranda is looking for a vessel that has a good physiology, so she gave up on her daughter, who, ironically, later began to look for a mother and family.
Of course, the organization Miranda mentions is The Connections. It was this crime syndicate that created Eveline in the early noughties. Miranda's rejection of Eveline explains why Mia and Alan were allowed to eliminate the girl on the ship. If Miranda needed Eveline, she would even forbid anyone to think about such a scenario.
But what about a rival company? Why didn't Miranda mention the HCF, since there were two organizations working on the girl? Either The Connections is a rival company, so it and the HCF are the same thing, or Miranda is related to the HCF in some way.
Why do I think the HCF is still involved in the story, even though the user who shared the leaks doesn't even know anything about this organization? Miranda can throw fireballs and transform into different types of arthropods during battle, so she is definitely related to the t-Veronica virus.
Speaking of Spencer, he and Miranda were clearly not in a business relationship. Their relationship was more than that.
I think the developers seem to have dismantled Spencer's castle from Resident Evil 3.5 with its fog, phantom and dolls, and then made these ideas into four "Spencers", that is, sinister aristocrats who create monsters and sow terror under the same symbol. Of course, they are all different and interesting as individuals, but the essences themselves connect them with Spencer at least on a conceptual level. Remember, for example, how long people thought that Alcina's castle was Oswell's castle.
And now we've found out that Oswell E. Spencer himself served Miranda. It wasn't a purely business relationship, because we know how Spencer feels about business partners. Miranda was his life mentor and, more importantly, these teachings began with Oswell's younger years. Yeah, you can find a photo of young Oswell in the game.
Now just think about it: this immortal creature named Miranda is surrounded by the canonical Resident Evil aristocrats. She is the one who "makes" these people, because Spencer's vision could hardly have come from nowhere. Everything has its own reasons, even if they are indirect. Of course, we hardly know anything about his family, but its influence on the founder of Umbrella has never been developed either.
But now we know that Miranda was an important figure in the life of an English aristocrat. A figure he respected and perhaps even loved, if Spencer knew love at all. When Miranda gave him all the knowledge, she predetermined his life, which led to all the events that we know, and also ruined his line.
And the most interesting thing for me is what kind of person this Miranda is. While people are trying to become gods to rule the world, the real god wants to be a mother to her daughter, who is her world. But why does she affect people so much, if we discard her divine image, which is not enough? What had she discussed with Spencer? What knowledge did she give him, for which he was grateful to her until the end of time? It's a very interesting perspective that I'm really excited about.
Most likely, the fungus evolved because of the Progenitor virus, so she gave Spencer a direction to find the key to evolution. And Spencer found this key in Africa.
Why didn't Oswell use the fungus before? To me, it's like asking why Dr. Marcus hasn't spent all his time studying a particular leech, if he has a virus that can make any leech special. The fungus has a very high "entry threshold", so even Eveline, who was observed from the embryonic stage, had defects. Spencer had the key to true immortality, which is much more powerful than the fungus, but he ruined everything.
Of course, at some point, Miranda could have made the fungus more powerful thanks to the t-Veronica virus, because the organization assisted her, but Spencer respected her, so he didn't control her or interfere with her work.
And this addition really explains why Spencer, using a book that was considered anti-scientific, believed in it and was able to predict the key to evolution. This is the part of Spencer's biography that has been bothering some fans, including me. I always felt like we weren't given enough context.