Will the one who unjustly says to someone, "May God never forgive you," be made to stand accountable in the the Hereafter, or will God not respond to him?

If there is no financial, reputational, or other injustice, then Allah does not answer the prayer of the aggressor, as Allah Almighty says: {Indeed, He does not love aggressors}. The supplication of a servant is answered as long as he does not pray for a sin or for the severing of kinship ties. As for the one who wrongs others unjustly, his rights will be taken from him on the Day of Resurrection.