> For example, bellum is nominative and accusative singular, bella is nominative and accusative plural.

That is true, but it's not really an example of Latin neuter nouns being subject to a fundamental rule specifying that the nominative case can never be distinct from the accusative case.

The rule exists, but the second declension forms are very standardized. You could equally use them to argue that Latin has a fundamental rule specifying that the ablative case and the dative case are always identical. (Though for that argument, the first declension would present a problem.)

Latin requires that neuter nominatives be identical with neuter accusatives even when the nominative form is unconstrained by its declension (only possible in the 3rd declension), and even when the nominative form is, by the standards of Latin, utterly bizarre. The neuter plural nominative of hic is haec (identical to the feminine singular, also an interesting [but not universal] pattern in Latin), and so is the neuter plural accusative. This is a word in the second declension, but it's a wildly illegal form for that declension.

And yes, as far as I'm aware, this treatment of neuters is an Indo-European universal rather than being anything specific to Latin.