grammar - Producing なる adjectives

I found this adjective from 7!!'s song 「ラヴァーズ」(Lovers):

無邪気なる

Dictionary entries categorises 無邪気 as a noun and な-adjective.

For reference, I shall extract parts from two different stanzas in the song. They share the same rhythmic structure and melody:

君は今 涙流した

泣きじゃくる子供のように

たとえ未来(あす)が見えなくなっても守るよ

夏の空見上げてニラんだ

---

君の手を強く握った

無邪気なる子供のように

たとえ時間(とき)が現在(いま)を奪ってもススムよ

夏の空目指して走った

Observations: The parts in bold both are 5 syllables and both end in る

Conjecture: The songwriter used 無邪気なる to be more poetic and lyrical. (Because without "る" it just "would not fit")

I understand that present day な-adj results from なる-adj which in turn results from classical Japanese's なり copula, and some なる-adjectives (like 単なる) survived.

(Question) Can we produce なる-adjectives by "regressing" な-adjectives? Are there any guidelines that prevents or allows us to produce なる-adjectives?

Answer

The answer to your question is simple.

We have this grammar in a lot of Japanese Christian prayers. For example we say:

王なる神様、this means Our God King
聖なる神様、our Holy God

Now there are some words that can be used with the なる form and some that can only use the な form

きれいなる女 is incorrect because the adjective きれい can not take the なる form.

There is no real way to tell which words can take the naru and which take na only but most cases the なる form can only be used with the おんよみ (Chinese reading) of a single kanji, like the two examples I gave you.

The funny thing is that 無邪気 can both be used as 無邪気な or 無邪気なる

The なる form is mostly used in songs or prayers or old Japanese more often, never used in conversation.