If you already have configurated your Local Network setting, so I recommend you to use the context instead of the variables as EXTERNAL OR INTERNAL_NET.
Examples of Context:
Context (In) [Internal to Internal]
Context (In) [Internal to External]
Context (In) [External to Internal]
Context (In) [External to External]
Answering your second question related to inherit, I believe that you are seeing the rule at Correlation Engine Level. If you want to change the overall setting, you should change this value at the root policy. (Local ESM/Physical Display)
But as I said, try to work with context instead, this setting will respect your Local Network setting.
Just in case, have you configurated the Local Network Settings?
You can configure "home net" in 2 places, that is the variable in the policy editor and the Homenet called Local Network that is a "setting".
Any IP addresses into Local Networks is considered "internal". This is used for many correlation rules context.
To configure Local Network: