But I do have a question: where is method Double1() defined?

You have to look into the source code for this one (either in Tk/HList.pm or Tk/Tree.pm).

I can't seem to figure out how to unhighlight an entry in a tree

To deselect an hlist or tree item programmatically, you have to use the selectionClear or selectionSet methods (documented in Tk::HList). This could be done in response to a FocusOut or Leave event.