This ought to be wrong in some languages

yep,
we need plural.
I cant see it translatable to sk for example.
Heal 1/2/5: Health
Lieč: 1 život
Lieč: 2 životy
Lieč: 5 životov
Although Health would be translated more like Zdravie but it would sound too strange in the context, anyway even that has plurals which will sound even worse, what means someone will probably change it in translation to život what is above or back to HP.

(Translators have the flexibility to do whatever they think is best for their language.)

Yes, but with singular translation they cant even to translate it correctly for plural parts.

rP23864
Out of curiosity, does translatePlural support the dual?

translatePlural works based on this: rP23690

They're hardcoded?

based on what transifex puts in po files.
Yes, we would need another library for it to not be hardcoded but they do not change so often,