Minimizing the risk of losing save data when playing offline

If you play the game in offline mode, make sure before you go offline that you have the latest synchronized data, then when you're going to play it offline, just play it. Once you go back online, if Steam prompts you that there is a "data predicament", simply choose the local version of the data. If you choose the data from the cloud, it will delete your local data (what you saved in offline mode) and replace it with the cloud data. So just don't choose cloud data, and you should have absolutely no data loss.