I came looking for exactly these shortcuts too.
In the case of ‘up’ and ‘down’ I think there’s some ambiguity.
‘Root/home’ is fairly clear — though whilst I go there currently, once saved searches / filter integration exists, I suspect that I’ll spend more time on some kind of saved search.
For ‘up’, perhaps a display like the Command Prompt could be used, to propose all of the parents as options to move up to if multiple exist.
For ‘down’, I’m not sure that I have any real request, since Find (Cmd+F) and Navigate In (Shift+Space) work well for me.
A shortcut for the cheatsheet would be nice.