Tried it - your screenshot looks different than mine. The lower wrench opens up the same menu as the top-right wrench. The 3 dots have only an “edit” pencil under them.
I edited one of my categories and selected “make all topics wiki by default”. That worked for that category. But there still is no way to wiki-enable one topic or to un-wiki a topic in a default Wiki-enabled category. Is there a setting I need to change?