For (1), perhaps add an if statement within the range?
{{ if eq .Params.categories "interaction" }}
... Do things ...
{{ end }}
Though I’m not sure on how to access the subdirectory off-hand.
For (2), you can either:
Make /layouts/_default/list.html blank (or whatever else you want it to be)
Which will affect all list pages.
Make /layouts/guidelines/list.html blank (or whatever else you want it to be)
You might need to create this to do so, but it will only affect everything in the guidelines section (that is, /guidelines/...)
Nested sections are not currently supported but are on the roadmap for the next release. Therefore content/guidelines/interaction/ and content/guidelines/ are both in the “guidelines” section.
That said, you could always filter based on a param that is unique to each of the content files under guidelines…