I haven't had a reply back from Shane on this yet but from what I've found, the only option to disable WD is by using the Group Policy Editor which isn't available in Win 10 Home - you need Win 10 Pro.
However, this article suggests a 3rd party program can do it for you if you want to give that a go, which may override what is greying it out.
http://winaero.com/blog/disable-or-enable-windows-defender-in-windows-10/You may also be able to disable the Windows Defender services by going Start - type services.msc and press enter.
Scroll down where you will see three of them.
In mine they are set to Manual, but I don't know if that is because I have Norton Security.
Click on each and a description of what each does will appear in the upper left pane.
Right click on each and select Properties then use the dropdown to change its status to Disabled - Apply - Stop - OK - File - Exit then reboot and then you can check its status where you would normally be able to turn it off.
I'm not sure if you would get any nags that it was disabled, but you may have to manually enable Windows Firewall to supplement Avira.