I haven’t investigated question 1.
For question 2, it seems xdg-user-dir DOCUMENTS is the query being run, in order to support localization of directory names. There are some ThirdParty binaries/libraries that also have dependencies on xdg-user-dir, so I assume that the answer to question 1 is “No”. (I imagine it’s also extracting the config dir ~/.config from $XDG_CONFIG_HOME).
As for question 3, I did install xdg-user-dirs and xdg-utils and so far have not had any problems on my headless server. Note that I have not run this extensively in a production environment, but for dev it’s working well AFAICT.