Is there a way to go to (open) the folder of a selected file when using the filter in flat view mode?
The command Go OPENCONTAINER will do that. It could be placed on a toolbar button (steps here or hotkey (similar but via Customize > Keys), or added to the right-click context menu for All Files (Settings > File Types).
Alternatively, if it is not something you need to do often, you can copy the file's full path to the clipboard (with the file selected, Edit > Copy Full Pathnames, or Ctrl-Shift-C), and then paste that path into the location field. It will open the parent folder and select the file.
Thanks leo! just what I needed.