So, the -c command was useless in my case, anyway. I mean I needed to edit the plist, then select Nvidia Web Drivers from the MenuBar or SystemPreferences, then reboot.
Graphics performance matter nothing to me anyway, I am not into gaming at all or into video or photo editing. I was satisfied with the minimalistic performance of what I had until now. I was able to run Garageband, that was enough, iMovie did not worked, but I did no cared at all about that. Giving another try with Nvidia WebDrivers would matter only if this made it possible for running High Sierra on this hardware, at the same level Sierra works now. I am asking because, even if I would be able to install WebDrivers and improve performance for graphics successfully now, I would still reinstall clean Sierra without them, to have it as it is now, only with Inject Nvidia, without Web Drivers. If I would be able to run High Sierra ok, I shall keep only InjectNvidia. Reinstalling the OS takes time and effort, so I am asking once again: Is it relevant for running High Sierra installing Web Drivers on my present Sierra installation?