[lentulusgracchus]

[wideawake] And yes, I know the real plural of opus is opera.

This is one of the irritating etymological truisms. There's no way in contemporary English to make any sense of it semantically as the two words have gone their separate ways. I won't even mention operas that have opus numbers.

Actually, the two words had already begun to go their separate ways in Latin. The third-declension neuter noun opus, operis, pl. opera, gave rise to a feminine first-declension collective noun opera, operae, pl. operae, with closely-related meaning. It's the latter word which is used in music today.

To make things even more complicated, all neuter plurals in -a were originally feminine collective nouns, hence the use of the neuter plural in Greek with verbs in the singular.

[wideawake]

Most of this is quite right but in one respect:

all neuter plurals in -a were originally feminine collective nouns, hence the use of the neuter plural in Greek with verbs in the singular

you may be overstating your case.

That's one theory as to why this is so common in Greek (and also in medieval Italian, Vulgar Latin and Provencal), but it's atheory.