Force automatic restart even with users logged in after installation of updates on Windows 8.1 Professional
This will do what you want with a powershell script.
download the powershell function/module from here https://gallery.technet.microsoft.com/scriptcenter/Get-PendingReboot-Query-bdb79542
Then edit the powershell profile for all users so that function gets loaded automatically. This link explains how to do that http://www.gsx.com/blog/bid/81096/Enhance-your-PowerShell-experience-by-automatically-loading-scripts
Then create a scheduled task to run the following powershell commands
$RebootStatus = "NotSet"
$RebootStatus = Get-PendingReboot | Select WindowsUpdate | Where-Object {$_.WindowsUpdate -like "True"}
if ($RebootStatus -ne $null) {shutdown -r -f -t 60}
Basically if the return value is false it sets $RebootStatus to empty, otherwise it populates it with a value. It checks that and reboots if $RebootStatus has any value other then $null.
Sorry if this goes over things you already know.