I've just spent another 45 minutes trying to install add-ons via Fennec ONLY. It just doesn't work. I only have access to "approved" extensions.
However, when I go to my custom collection in a desktop browser I have full access to firefox addons (as you described we would)--but I have to first search and add them to the collection from the desktop browser.
When that's completed, THEN I can go to Fennec and navigate to the collection and tap the "+" sign to install them into Fennec.