As already outlined there exist some macros quite helpful to start off.
The Initial Code Fragment
[Error: Macro 'code' error: code() missing 1 required positional argument: 'ext']
First Problem Solved
Even if adapting the page to my system this piece of code yields an error, which is caused because the variables environ['wiki.metadata_page'] and environ['wiki.page'] are initially not defined. I found a work around which allows to set the variable to the current page value if empty.
[Error: Macro 'code' error: code() missing 1 required positional argument: 'ext']
A much better solution is excluding the recursive test from the first page.
[Error: Macro 'code' error: code() missing 1 required positional argument: 'ext']
Downscaling The Included Text
Next I wanted my included text to fit the structure of the parent page. That is all subheadings have to be downgraded, so I created another parameter that signals the level at which the included page resides.
<<include slug=WaddingtonExperiment headerlevel=2>>
My include routine receives the parameter accordingly.
[Error: Macro 'code' error: code() missing 1 required positional argument: 'ext']
Finally for this Genshi filter to work I had to create a new class.
[Error: Macro 'code' error: code() missing 1 required positional argument: 'ext']
Probably this piece of code looks a bit awkward to those more experienced in python programming but it works. Unfortunately I didn't find any documentation how to create a filter and how the data structure in that filter look like.