Saddle seems slightly lower that needed on the video. When you take off your hands of the bars I would bring them behind your back(do it on the trainer). It is called a balance test. If you keep falling/sliding forward I would tilt the saddle up and maybe slide it slightly back, experiment with it. You should be able to ride on the trainer without sliding forward with your hands behind you back and still keep the normal riding cadence. It should help with the pain in your hands. As far as lower back pain when riding hard most likely core strength is not enough as mentioned above.