Why not cut down to the new plate to X level on the inside and then its where you can weld it. Sure, not as good as getting to the top corner but seems like a decent compromise.
1 piece or 3 piece would be about the same if everything is perfect. The 1 piece you have no adjustability so if your having to repair the frame at the same time so you might then have to cut material off the end of the crossmember if it interferes, or end up with a larger gap than you want for welding.
Good question, lets break it down. "Can you do this with a life?" Really depends on the life, some life is hard and complex so it may not allow you to get this done. Some life is soft and free-spirited which means there may be life available to get this done. "I don't have access to a life" Unfortunately we don't have life for sale, you might check Amazon though maybe you can buy life there.