Sorry for the late reply ...
The Metacello interaction with Monticello repositories is complicated
... there are rules within Metacello about whether or not the mcz file
in the package-cache can be used or not and without the error message
for the load failure I couldn't guess what might be going on, especially
in relation to a ConfigurationOf, since Metacello will not trust the
package-cache when accessing a ConfigurationOf when it decides that it
needs to download the latest version of the configuration --- which is
driven by another set of rules regarding whether or not the version you
are requesting is present in the currently loaded ConfigurationOf ...
If you want to truly independent of the network, your best bet is to use
local clones of git repositories for all or you projects it's a bit of
an effort to do this for those projects that haven't migrated to github
yet, but using git and BaselineOf is the best solution ...
There are ways to avoid the network when using ConfigurationOf ... but
they are a bit more complicated to setup and maintain ...
Dale