for the home dir issue there is:
regarding $EDITOR, snaps do transparently support xdg-open to open files with unconfined apps outside of their confinemet.
you could try to set EDITOR to xdg-open in the environment of your app in snapcraft.yaml, that would then (hopefully) hand the file path to xdg-open outside of the confinement, which in turn should run whatever application was defined for the mime-type of the file (an editor for txt files hopefully 
)