(Sorry for the late response, I've been ignoring my generic email because it's full of junk.)

You can configure the connection for SQL configuration several ways, including environment variables. What's important to keep in mind are the ways you can't configure them.

You can't configure the configuration connection with a command line switch, as this switch will not be processed until after all the configuration settings have been read. Ditto for parent package configurations.

Any other direct configuration technique (environment variable, XML file, a separate SQL configuration) can modify the connection manager to control the source for later SQL configurations.

Expressions on the SQL config connection manager will not be evaluated until after the configuration settings are read, so the connection manager properties must be configured directly and not through a package variable.