There is not way to find out the template path of the base object. You will have to copy the content of the base export template to the template of your derived object. I know this means you have to update your own object if there are relevant changes in the base object, but I don't see any other possibility.
Your understanding of object derivation is correct. Just some more hints:
The elements in the UseAs and Templates section replace (override) the inherited ones (like you said).
All elements in the Content section are usually aggregated, but with one exception: If a variable or type is referenced by a UseAs element (e.g. the Extension), and this UseAs is overridden in the derived object, the variable provided by the base object is ignored, so only the Extension from the derived OOD file remains in the project. In case of the Unit and Extension, also the type definition from the base object is ignored in case it is instance specific.
That means if you don't provide a <UseAs><Extension> in the derived object, the Extension variable and type definition from the base object are created (just as any other content from the base object).
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