@ Goku knocking Black's A$$ back to the future and Black still feeling that pain 17 years later, LOL. Goku basically punished him in Super Saiyan Level and not God Status Vegeta was telling Trunks how can you not beat him in Super Saiyan 2?. Also LOL @ Goku and Vegeta fighting about who's going to kill Black and then Goku hurries up and jumps in the air to fight him before Vegeta. Black could only be in the past for a limited time. His time ring only lets him Time Travel for but so long. But Black's fight skills were hard body. He knew Vegeta, Whis and Beerus so the idea that he just posseed Goku's old body might be true. He also grabbed his heart in the future which means he could of very well inherited Goku's Heart Disease in that future Goku Body.
Black Destroyed Trunks Time Machine to trap him in the past. Kuririn asked Whis to help Trunks go back to the future and even remined him about bringing time back during the Resurection Freeza Battle. But he explained that he could only go both back in time or in the future for about 3 minutes or so. As for Trunks Time Machine that Cell used in the future. Remember the Z Fighters discovered it in the forest when Cell went back in time and it was never destoryed. I can't believe Bulma remembered it from back in the day. It was really good writing and it connected to the Cell Storyline very well. Based on the preview it looks like Trunks will go back and fight Black and what a surprise ending with Mia. But yeah Goku gave Black that work and was only testing his strenth levels. Also I thought it was interesting Goku called Black "Omega Black" as in he's the Omega to Goku's Alpha.