Yeah, that dpkg command won't work without sudo, but I don't think it will do any more than apt-get purge.

Try this:
sudo apt-get purge texworks
ls /etc/texmf
If it has successfully emptied the /etc/texmf directory (with the exception maybe of fmt.d directory), then I'm stumped. If all the same files are there as before, proceed like this:

sudo cp -rp /etc/texmf /etc/texmf.old
sudo rm -r /etc/texmf/*
sudo cp -rp /etc/texmf.old/fmt.d /etc/texmf/fmt.d
sudo apt-get install texworks