As it happens neither Lessing nor those he is criticizing think that Leibniz actually believed in eternal damnation in hell. All of them offer reasons why the arguments clearly in support of orthodox views of hell in Theodicy should not be taken at face value. But why the apparent endorsements at all? One Eberhard thinks Leibniz can't help himself, craving the approbation of all and so thinking of crafty ways of marketing his system as compatible even with the most errant views. Lessing thinks this sells Leibniz short. What Leibniz is really doing is what ancient philosophers did: meeting their interlocutors where they were, finding the grain of truth in their otherwise erroneous systems (for there is no view actually held be someone which doesn't have some truth to it), and guiding them with its help towards more truth.
Leibniz's own view, Lessing argues, is that the doctrine of eternal punishment contains a valuable truth: everything is connected, and so every act has infinite consequences. Since God has so arranged nature that evil is punished (the agent's perfection is diminished), every sinful act produces its own infinite series of punishments. That's it! No need for a hell at all, just immortality. And for all we know the infinite duration of punishment is compatible with, even constitutive of, a kind of restoration too. So you can have your universalism without giving up philosophical rigor if you work with rather than reject tradition. What's not to like?