Well, before anything else... Did you select the "beta branch" that kept the client to Patch 4 instead of updating to Patch 5, as the update explicitly told you to do if you wanted to keep playing the same version?
Otherwise you wouldn't be able to reload your old saves to begin with.
That aside, you can sidestep the game launcher entirely by executing the game directly from its executable with Steam closed. But I'm not sure if that allows for multiplayer.
