And that works except that, in the page editor, the "in between" parsys containers of that resourced content fragment component are visible but components can't be added from the component drawer. If I drag them onto the page over the parsys, they don't drop.
dragging an image component into parsys but it is disabled
I don't think so you can author content fragment's content in the page editor. You can configure and browse content fragment which is created using content fragment model. Are you not able to configure content fragment when you include from HTL or the page parsys is become inedible because of CF?