(Why was Hitler's free will more important than the free will of his six million victims?)

Hmmm. . . i will not answer this question because this is not the first time God has done what He must to His people. And He warned them many times. See Old testement. deuteronomy 28 verse 15-68. Remember that the jews have a covenant with God that says He will do this to them if they do not obey. Which they agreed too.

(Why doesn't god simply take people to heaven the instant they convert? That would prevent backsliding and save more souls.)

What would stop you to backslide in heaven like Satan? And what if you reform in hell? Till jughment day you can still chose between good and evil as we know it.