Is cruel standard use as a noun in poetry? Are there terms for non-standard English specifically in regard to use in poetry?
Solution 1:
My guess would be that cruel is used to indicate a person or something personified: oh, cruel one—not cruelty.
1.) If a poet uses uncommon instruments confined to poetry in order to fit the metre/rhythm of his verse, he is said to be using them metri causa, "for the sake of metre" (Latin). I don't think there is any more specific term to match your examples.
Using a double negative to mean a (strong) negative is now just slang, which can be used as a figure of speech.
Using a double negative to mean a (strong) positive is called litotes:
There was no lack of willing maidens.
2.) Omitting letters (mostly vowels) in the middle of a word is called syncope.
3.) [Edited:] Assonance means simply that you use two words or syllables in close proximity that (only) share the same vowel sound, as opposed to consonance, where they share the same consonant sound. If they share initial consonants, it is called alliteration.
Assonance: of lock and pot
Consonance: all the levels will collapse
Alliteration: seven sodden sisters
Your example ev'n–heav'n would be consonance; there might be a word for semi-assonance, but I don't know any. I'm not sure what to make of come–one–home: it is conceivable that their vowels were identical in 16th-century pronunciation. Even and heaven might even have rhymed in the 16th century—I don't know.
Solution 2:
No, it is not common to use "cruel" as a noun; but I think it is actually being used more or less as a name.
Solution 3:
This is very poetic usage, where anything doesn't always 'go' but there sure is a lot of leeway. The poet probably didn't think the noun form 'cruelty' word fit the meter.
So it would be ungrammatical in most any variety of everyday spoken English to use 'cruel' as a noun, even personified.
Poets take liberties with all sorts of things, pronunciation, syntax, word meaning, etc., in order to fit some arbitrary meter or rhyme scheme