Couldn't install SSMS: A pending restart is blocking setup from completing
Go to the Visual Studio Isolated Shell download page, fill out the form, and ask for permission to get VS Shell.  Download Visual Studio 2015 Shell (isolated), run it, repair, reboot, run it again, uninstall, reboot.

Now, SSMS 17.x should install without errors.

Alternatively, (and less safely): Search the registry for "visual studio 2015" and delete all the keys that are related to the shell or its prerequisites. Repair SQL Server 2016 and then install SSMS.