I completely agree that we tend to judge in hindsight rather than by the evidence that was available at the time -- that is one of the diseases of Currentism. But in this particular case... I dunno,
Mein Kampf was published in 1925, so
all the evidence one would need was there, and not subtly put forward. Perhaps the West is somewhat exonerated because they literally couldn't believe those psychopaths actually meant it. But as you point out, to let the Jews come "flooding" into the US would have provoked easily exploitable narratives about The Menacing Other. As so often happens, the "leaders" followed and didn't stand by our true values.
Or to put it another way, if Iran occupied Israel tomorrow, I'd hope we'd all agree to let all the Jews emigrate here, immediately, no boarding pass required. And we probably would. But if the Sunnis decided to rid the world of the Shiites once and for all, try selling that in Peoria.