Regarding hiding depression during application
You are under no obligation to share your medical history in an application, but you are under obligation to be truthful -- so no lying about the date of your accident.

It seems like you actually made the best out of your decision to drop out of the program back then. It's legitimate to say, in an application, that you felt at the time that the program was not the right fit for you, and that you wanted to pursue other opportunities -- which you could then describe by saying that you founded a start-up and worked on your MSc project on the side. If you phrase this right, it actually looks quite good, and it's not even a lie: You did leave to pursue other opportunities.