Cross-save and the Steam migration are two separate things. Whatever characters and game licenses are on your Battle.net account, is what will be migrated to Steam. That means you will only see your actual Battle.net characters when viewing the transfer data. Cross-save allows you to link your various accounts, and choose one of them to be the sole save data that will be playable on all platforms. This includes your characters, and in-game inventory. It does not include game licenses, as you must purchase the game licenses for each platform you wish to play on. Cross-save also does not delete the characters and items from the overridden accounts. The save data for those accounts is simply made dormant while cross-save is active.