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I can't take seriously the scholarship of a writer who seems not to appreciate the difference in Greek between a singular noun (ἀπόκρυφος) and a plural noun (ἀπόκρυφα).

That is a supremely stupid objection.

"The apocrypha" is the typical way of referring to the aggregate of "the apocryphal" books (adjectival term) in English. When inserting a Greek term, you use the Greek. When writing English prose, you use English. Not that difficult of a concept.

For other similar uses of the term "apocrypha/apocryphal, see:

New Testament Apocrypha: Gospels and Related Writings, by Wilhelm Schneemelcher

The Apocrypha in the Irish Church, by M. McNamara

The Oxford Annotated Bible, with the Apocrypha, eds. H.G. May et al.

Introducing the Apocrypha: Message, Context, and Significance, by David A. DeSilva

The Septuagint with Apocrypha: Greek and English, ed. L.C.L Brenton

And so forth.