Again, that is simply not true. At absolute most, Many-Worlds theory means that anything that is possible will happen in some universe (and even that isn't certain, because there has to be some set of quantum events that could lead to it). It certainly does not make the impossible possible. Even if you have an infinite number of worlds, something that has literally zero chance of happening will still happen in none of them.
Well for becoming a HS Shirou would have to do something important enough to become a legend. He would also need to have been lived during the time Jack the Ripper, Li, or the AoG since modern day people cannot become HS. Even people with completely unique abilities or magecraft like Zeltretch or Aoko cannot be HS and Tracing is not on the level of the True Magics.