You're on the right track.
Old English had a phonetic property called intervocalic voicing, where a
fricative consonant—f, þ, or s—became voiced when it occurred alone
between two vowels: sē stæf ("the staff") was pronounced [seː ˈstæf], but
þā stafas ("the staves") was [θaː 'sta.vas]. This was, in fact,
the only way to get a v, z, or ...
Although I can't provide evidence to support this, my feeling is that "apple of my eye" is not syntactically flexible. The pluralization of 'eye' (Sentences c and d) definitely sounds wrong. The pluralization of apple in Sentence b might be acceptable to some native speakers, but it sounds wrong to me. The only sentence I would consider natural is ...
I would add that in most forms of everyday conversation all that is needed is never referring to poetry unless in educational settings.
"The, that lyric" for specifying one line or stanza of a song
"The, those, lyrics" for an entire song
Almost any uncountable word can be countable.
I can talk of cheese and cheeses (Cheddar and Wensleydale); of bread and breads (white and soda bread); even water and waters (pond water; sea water; drinking water). I can have a number of wines in the cellar. Opportunity is like this: many opportunities can arise to do something, in which case you have a lot of ...
Your example is fine as written. Imagine the following exchange:
You have lots of clothes you can wear to the party, right?
No, the only thing I can wear is my shoes.
In your example, the singular verb "is" agrees with the singular subject "the only thing..." (the verb agrees with the subject, not the complement).
The complement "...
"The phrase apple of my eye refers in English today to something or someone that one cherishes above all others. Originally, the phrase was simply an idiom referring to the pupil of the eye."
"Your eye (that is, you yourself) can have only one ‘apple’.
The apple of one’s eye is the delight of one’s life, the thing ...
The idiom can be considered both (1) to be extra-grammatical (here hijacking a reasonably common if far from simple sentence structure and using it in a strange way) and (2) to be using words in a strange way.
We'd expect a weather-it sentence to have one or more adverbs or prepositional phrases etc modifying the verb:
It's raining continuously. / It's ...