A collapsible tree menu won't work in the topic simply because there is no real hierarchy of resources/activities in the topic. The indentation is merely a visual effect. All the item links reside in one flat list.
The desired cleanup is only an issue if you have several chapters such as the one illustrated above on the same page. But you can have one chapter (topic) per page and then you don't really need to collapse the items.
The one topic per page has just been added to the forthcoming 2.3. In the meantime I think there is a released contributed format which does that (I have developed such a format but it has not been released yet).