The Mold works exactly like the Progenitor Virus, because it probably evolved because of it. Resident Evil Village did not invent new rules, but only approached the potential of the virus, on the basis of which Umbrella specifically tried to create a strain that is hundreds of times weaker and that is compatible with modern organisms and Spencer's body. But even the primitive t-Virus allowed the leech to become Marcus with his memory.
You say the C-Virus makes more sense, but it can also resurrect dead cells and store Ada Wong's DNA, making even a swarm of bugs able to mimic her shape. The whole biology of the series is bound by general laws, people just like to think that the series used to be more realistic and logical, although all that has changed is that now the developers have a free hand, as they have been planning crazy things since the 1990s, but could not implement them for technical and other reasons. They even used the old ideas in the new game.
No one knows yet whether Resident Evil 9 will be a game about Rosemary. Even the new game itself shows that there will be something between these events, because the story ends with a plot twist about the BSAA and Chris' order. The finale with Rosemary is there in terms of plot, to complete the storyline and show what kind of future Ethan has given her. Accelerated growth doesn't make any sense and even necessarily dark.
