[Ultra Sonic 007]

A Jew converting to Christianity would have to agree that his beloved ancestors are in hell but that he or she hopes to do better.

Not quite. From the Third Chapter of the First Letter of Saint Peter:

18 Because Christ also died once for our sins, the just for the unjust: that he might offer us to God, being put to death indeed in the flesh, but enlivened in the spirit, 19 In which also coming he preached to those spirits that were in prison: 20 Which had been some time incredulous, when they waited for the patience of God in the days of Noe, when the ark was a building: wherein a few, that is, eight souls, were saved by water.

Between His Death and Resurrection, Jesus descended to the realm of the dead, bringing His message of salvation to those who were dead and yet not damned to Hell. So your argument doesn't stand.

[Ken H]

What about more recent Jewish ancestors such as deceased grandparents, deceased parents and so forth? A Jew converting to Christianity would have to accept that their deceased relatives could never escape eternal damnation.

How could anyone be happy or at peace knowing that their beloved relatives would suffer forever?