Further to this, today's Cumulative Update 2018-02 (KB4074588) has installed successfully on my device with the "requires restart" error. However,
in updates history KB4058258 no longer shows "requires restart" but instead "failed to install", with error 0x80070643. I note this is (still) listed under known issues for these recent updates, as a false report . Happy to assume all is actually now well, at least until 1803... happy enough not to contemplate reinstalling the OS, anyway.