Is my "solution" safe and appropriate? Should stable be included in my sources.list permanently?
It is pretty safe and I would recommend leaving Stable in "sources.list" because of security updates. You can set the default release in "/etc/apt/apt.conf" like this:
APT::Default-Release "release";Of course you should investigate why it was removed in the first place. Maybe it was replaced by some other package or was too buggy. Just a word of warning.