I had exactly the same problems since January. I did the "SC Config Trustedinstaller Start=Auto" then ran WUShowHide.diagcab to release KB4088776 that was hidden then Restarted. The system came back up and WU started downloading again but took much longer than before. Then it automatically Restarted and began installing ALL the updates for the past 2 months. It took over an hour and automatically Restarted numerous times. When it finally was done WU Installed Updates still showed KB4088776 as having failed with (6) tries but Uninstall Updates show 5 installed updates with todays date and KB4088776 was the most recent. Check for updates says I am up to date.
Using Task Manager Services tab I checked Trustedinstaller and it says Stopped. So I am not sure if the "SC" keyin will have to be done again or not. But that is what I will try if WU gets hung up again.
Thanks