Regarding Jesus’ teachings about ‘gehenna’, I think all agree that his language is metaphorical (i.e., Jesus was not refering to a literal valley of perpetual fire outside of Jerusalem).

Actually, I don’t agree that his language is metaphorical. When Jeremiah describes ‘the dead bodies of this people’ piling up in the Valley of Hinnom (= Gehenna), he is not speaking metaphorically - he is describing the consequences of military action against Jerusalem as a consequence of the evil which Israel had done (Jer. 7:31-34). Jesus certainly foresaw a similar catastrophe for Jerusalem - why then should he not be using the same language in the same way? Because of the wickedness which Israel persisted in doing, the bodies of the dead - killed by the Romans or by starvation and disease - would lie burning the valley of Hinnom.

In Mark 9:48 Jesus adds an allusion to Isaiah 66:24, which concludes a prophetic account of the restoration of Israel following judgment and the defeat of Israel’s enemies. The point of the image is that those who survive this devastating day of the Lord will look upon the bodies of those who rebelled against YHWH (the dead rotting outside the walls of Jerusalem) with abhorrence.

Here the language of new heaven and new earth is used as a metaphor for restoration. Whereas in John’s vision of a new creation there is no more death (Rev. 21:4), Isaiah only promises that ‘the child shall die a hundred years old’ (65:20). I don’t think you can conclude from this that when Jesus uses the same image he is speaking of a universal judgment. Notice also that the dead are simply dead - they have suffered at the hands of their enemies but now they are only corpses. You cannot torment a corpse.