Re: How I search only sub directories in my bookmarks?
If the sub directories do not have other sub directories or all sub directories are open then you can use a workaround in the sidebar (View > Sidebar; Ctrl+B) and select that folder with a left mouse click at the far left to set focus to the bookmark area. If you quickly type the first letter(s) then a search is done for this text and the first item found is selected. You will have to type quite quickly and you can only search for the title and not the URL. Repeat this to go to the next result. This doesn't allow to get a list of found items as you can only go from one result to the next.
I don't think that you can do this with a place query either.