[kbs666]

There is a setting to globally clear all revealed content. That would be the first thing to do if I was you.

Associating mechanics and topics is best done by linking one to the other.

To export a subset of a realm is easy. Go into the export dialog and choose partial export and follow the directions. When you're done you should have a new tag specific to that export. Create a view and add only the stuff you want to export, and eventually delete, to it. If it's all under one topic add that topic and its children, which should be an option. Then go into bulk tagging and assign that tag you made to everything in the view. Go back to the export dialog and do the actual partial export.

Once you have the XML file, I'd import it into an empty realm just to be sure the data is there. Once you do there is no reason not to delete that source and everything under it from you main realm.