It has absolutely nothing to do with either KFMon, NickelMenu, KOReader or Plato, it's entirely Nickel, so uninstalling anything won't really change much
.
What you *can* try is zapping the entire `FeatureSettings` config section, making sure you don't actually have multiples of it and/or of the `ExcludeSyncFolders` key; then eject, reboot and try an OCP install again.
You might want to try removing the KoboRoot.tgz (you don't actually need it, and that'll skip the reboot and hopefully leave Nickel time to grok WTH is happening) *before* ejecting that final time, though.
Another option instead of re-running the OCP is to just tweak the config file yourself, that's effectively the only thing not working right now.
(You'll still have to reinstall KOReader/Pato once Nickel has deleted those PNGs to restore them, though).
But before all that, I would also run a database integirty check via the calbre Kobo Utilities plugin, because, if for some reason your Nickel db is borked, that *could* explain your issues.