I assumed you only wanted to know about the command-line switches as a last resort, if there wasn't a better/built-in way to do what you want.
Also, as explained in this FAQ, it's best to tackle one question at a time (especially if the second question may be irrelevant depending on the answer to the first) to avoid threads becoming interwoven and hard to follow.
The Customize -> Buttons list has pre-made buttons you can put on your toolbar, but you can also create (or modify) buttons which do different things by editing them, which allows you to pass arguments which change what they do. (Each button has a script inside of it, effectively.)
The Toolbar Editing tutorial is a good way to quickly learn the basics of button editing.
To see what the Favorites command can do, open the Opus manual (F1) and then go (near the end) to Opus Raw Commands / Favorites.
I'm not sure if the Favorites command will do what you want as I'm still not sure exactly what you want. It won't literally put a tree control on a toolbar, but you can have it list your favorites on the toolbar, either with pop-up menus for the sub-levels or as a flattened list (by inserting multiple commands to list each level).