It seem to me the Editable Templates are useful for a single site usage as design/policies configurations are defined on template level. i.e. imagine to identical sites in structure but they have different design/policies configurations for structural components (ex. header, footer), then in this case you will need to create to set of identical editable templates to be able to define different policies for the structural components on those sites, which does not seem like a beautiful solution to me. Are there any other ways for defining different policies for structural components per site on the same editable template?
In static templates world the design configuration are fetched from the designpath which is configurable per site, but this is not the case here, unless it is possible to define policies per site,
We had a similar issue with MSM sites where we had a requirement of same header with different contents. So we come up with iparsys solution along with editable templates. To achieve this create iparsys node below structure node in /conf where your template is/are presents.
Then use these iparsys header and footer to drag and drop your components. With this you can cancel or disable the inheritance as in required.
Policies/design dialog values in static template are template level. If you have another template then you can set another policy. For structure components you can rely on experience fragments core component which support MSM(header footer will change automatically based on language).
In your case iparsys could be the solution but it does not work with dynamic templates.
To have different header, footer or another structure components for same templates you need to write an custom components to get inheritate values or read only from root page and default authoring can be done from as template's initial.
There is one more solution which is context aware configuration where you can specify structure components and content but it could be more hard coded since there is no configuration editor available as of now. but you can create pages inside conf.