EDITED
To say that ‘ only a prophet kew that there would be a war’ flies in the face of all that we know and knew. The Nazis had been in power for seven years and the expulsion of jews from germany and Austria was going on without stop. it would have been very prudent for people to look at alternatives in leaving Europe and indeed many did. Virtually everyone was expecting a war.
No one has ever accused rabbonim and Roshey yesihiva in being-G-d forbid- complicit in the Holocaust, but also, you cannot make them infallible and know-all. Thye do NOT know all and they sureyl CAN maker mistakes. This is the essence of the difference here and in the world. R”elchonon was overly worried about the spiritual danger of the US and this was his right, regardless when he wrote this letter. This was also the position of many other gedolim but they did NOT foresaw what as to come and this is the rux of the matter.
gedolim are NOT infallible, rabbonim are not know-all and they should be treated as such. They can err.
As far as Kasztner geos ,enoug has been written about this and there is little to add. Maybe the nazi he defended was indeed good to him and to others? there were a few,very few who did not participate in the murder of the jews. maybe he was one of them?