These points are ALL MOOT. If we consider the fact that time travel is possible, the energy required to achieve the speed required to travel back in time to witness the event of multiple american presidents engaging in fisticuffs is far too big to achieve with current technology, and also, this scenario requires travel forward in time to be possible. Hypothetically speaking , I theories that it might be possible after accelerating past the speed of light and to a speed where the object travelling back in time, would be travelling back in time at a fast enough speed to negate the effect of going back in time and instead would propel you forwards in time. However, as I have already said the energy required for this would be immense and the process of travelling forwards in time is only a theory which means that it probably is not even possible. I am sorry to have to crush you dreams this way but scientifically speaking, a scenario where multiple presidents from different periods of time fought each other is only possible with cloning, and as almost all remains of some of these presidents are gone, even that is doubtful especially if you consider the fact that cloning is a very taboo subject in modern times, with some scientists being banned from even trying to clone people.