We had the same issue with the install not finishing despite everything being installed and the Installer.exe no longer running, found that if you end the 'AdskAccessService' process it then finishes. We added the following lines to our PowerShell install script to sort that out:
do {Start-Sleep 10}
while (Get-Process -Name "Installer" -ErrorAction SilentlyContinue)
Stop-Process -Name "AdskAccessService" -Force
i.e. check if Installer.exe is running every 10 seconds, if not, end AdskAccessService.exe, install process then actually ends.