Is it sufficient to do this?
sudo -u bitc HOME=/home/bitc dconf write <...> ^^^^^^^^^^^^^^^
I was trying to do something similar. I have a script that connects via SSH and runs as root. I want it to change some settings for a user. This seems to work for me:
sudo -u vagrant HOME=/home/vagrant dbus-launch --exit-with-session gsettings set org.freedesktop.ibus.general use-system-keyboard-layout true
I think that's potentially bad because it's spawning a new dbus-daemon for the duration of the this session, but without the dbus-launch I got dbus errors. Perhaps these are specific to gsettings, but I want to mention it in case it helps.