What do I do if I have killed Trevor before starting the Real Estate mission

This happened to me once. If you press start while playing the game and tab across to the game option you can load from a previous save. In my case I had created a save and somehow my xbox date was incorrect meaning the game loaded from the latest save which actually had less percent completion.

It'll be because the saved game is strictly for your profile (your gamertag), you won't need the internet to save, but you first need to download your profile to continue the saved game.

Having your cloud enabled can also cause this problem - try disabling it in Storage settings.