Opening the game by testing a room in InstaPreview mode and exiting the run (not the game!) will put you in the menu of your last selected save file. If that save file has a run you can continue and you do it in this state, it will completely wipe that save file.
Sorry there's not much we can do about that on the BR side: that aspect of instapreview is implemented by isaac itself. Perhaps repentagon could patch this?
Opening the game by testing a room in InstaPreview mode and exiting the run (not the game!) will put you in the menu of your last selected save file. If that save file has a run you can continue and you do it in this state, it will completely wipe that save file.