If I shot in "Velvia", but decide later that "Provia" would have been better, why can't the camera switch the "Velvia" setting to "Provia" in the RAW file? It is just a setting within the RAW file. Overwriting the original parameters is easier for me because I don't have to bother with additional files on a computer. Just let me save those parameters. The image won't be altered, just the parameters used later by the converter to generate the image. For me that would make sense. I don't want to lose my time because I have to boot a computer where some software is running. It could be done within seconds in the camera.
I have owned several X100 models most recently the x100v and also the Leica Q2 and Q3. In my opinion as a 20+ year enthusiast the comparison is apples and oranges. Both systems have their strengths. If you love the Fujifilm system or are mainly a jpeg shooter, the x100vi is the one to buy. When I initially purchased the Q2, I quickly realized that this was intended for post processing. The jpegs were acceptable but not as robust as Fuji. The Q3 is solidly built with simple controls, tilt screen, an intuitive menu system, Summilux 28mm lens, 61 megapixels, weatherproof, IBIS, improved low light performance and dynamic range. If the Q3 is within you budget, my suggestion would be to try (or purchase) both and decide what fits your needs and style.
Your last point does not make a lot of sense. It is similar to saying ‘take a raw file, run it through LightRoom or whichever raw convertor you are using to find a neat version but then overwrite the saved parameters in the original raw file instead of saving the new version as a separate image.’ Raw files store the image as shot. Using what you are suggesting, if someone needs to go back to the original ‘as shot’ image, it would be impossible.