"control+down arrow" should uncollpase the children items of that one list.
"control+up arrow" should collapase the children items of that one list you are over.
"control+right arrow" should zoom you into that item (as intuitively going to the right)
"control+left arrow" should get you out of the zoom and back into it's parent item.

Also it is needed a double click in the parent item to collapse all / uncollapse all sub items. this is fundamental!

Please refer to how WorkFlowy do all these shortcuts and hotkeys which is perfect and it allows you to navigate and make every move just using the control+the arrows keys.

It's really intuitive, logical and saves a lot of time!

Thanks!