Hello,
The current DHCP DSC resources from Microsoft don't use or configure the failover feature.
You can have a look at what it does on github using this link
https://github.com/PowerShell/xDhcpServer
It means that these resources won't mess with the failover configuration and if you want the DSC resource to handle the failover configuration, you'll have to write your own custom DSC resource.
For your 2nd question about whatif someone changes the scope. The answer is twofold.
If this person changes the configuration and applies it, no problem, that's the purpose of DSC.
If this person changes the configuration in the GUI and ignores the DSC configuration, it depends. If the LCM (local configuration manager) is set to 'ApplyAndAutoCorrect', well the DSC will restore the scope into the desired state defined in your configuration.
If it's set to 'ApplyAndMonitor', it will only register the drift. If it's set to 'ApplyOnly', the LCM does nothing and what was changed via the GUI remains. The LCM properties are documented on this page:
https://technet.microsoft.com/en-us/%5Clibrary/Dn249922.aspx (the description of the LCM behavior I made assumes you don't have a pull server)
