There really isn't a way, at least currently, to show a list of games that have been combined. This would be a good field for the Audit feature though, so feel free to request that by clicking Issues/Requests at the top of the page.
If you re-import games and use the auto-combination, then the games will re-combine based on what LaunchBox sees. If you remove the games you'll have to re-do things like playlists (unless automatically populated), favorites and anything you had to manually do that LaunchBox doesn't do.
When you delete a game in LaunchBox, it only removes the game from your platform and removes the info on the backend. You can also turn on the option to delete the rom file associated with it as well. It's an optional setting that will ask you after you press yes to remove if you want it to remove the file too. If you re-import that same game though, it should go trough just fine. The only time LaunchBox wont import a game is if the exact file is already located in your LaunchBox library. So if you removed half of a platforms games, then ran the import for that platform, it will import the games that aren't currently imported.
If you need more help feel free to ask!