You're describing a special case of a special case. Let's start with the less special case.
Words like 'die' and 'cast' that have multiple meanings are called ambiguous, which means:
(Of language) open to more than one interpretation; having a double meaning.
Given that its constituent words are ambiguous, your example of "the die is cast" can thus be said to be an ambiguous proverb. This phrase even has some currency, evidenced here and here.
Now, ambiguity can be caused by the word having multiple meanings, in which case it is called lexical ambiguity, or it can be caused by there being multiple ways to group the words in the sentence, in which case it is called syntactic ambiguity.
Your example of "the die is cast" is an example of lexical ambiguity. A few other examples of lexically ambiguous proverbs are mentioned here:
- A good husband makes a good wife.
- Good masters make good servants.
These are ambiguous because "make" can be interpreted as both "bring about" and "amount to". Notice that, unlike in your example, resolving the ambiguity does result in different proverbial meanings.
Now, your example of "the die is cast" is a special case where the ambiguity in the constituent words does not scotch its figurative meaning. In that sense, you might not want to call it an ambiguous proverb, since regardless of the disambiguation, you arrive at the same figurative meaning.
Instead, to coin a phrase, you might call it a bistable proverb, where bistable means:
having two stable states.
In a way, a hearer who encounters this proverb can vacillate between the two literal disambiguations without it affecting the figurative meaning. The figurative meaning is thus bistable.
Finally, your example of the (bistable) proverb "the die is cast" is very similar to examples of so-called eggcorns, defined by Wikipedia as:
In linguistics, an eggcorn is an idiosyncratic substitution of a word or phrase for a word or words that sound similar or identical in the speaker's dialect.... The new phrase introduces a meaning that is different from the original but plausible in the same context, such as "old-timers' disease" for "Alzheimer's disease".... Eggcorns often involve replacing an unfamiliar, archaic, or obscure word with a more common or modern word ("baited breath" for "bated breath").
In an eggcorn, words are substituted in a phrase for similar sounding words. The result is an expression that, although different in literal meaning from the intended expression, has roughly the same "figurative" meaning. For example, "old-timer's disease" for "Alzheimer's disease".
Your example of "the die is cast"/"the die is cast" might be considered a limiting case of eggcorn, one in which phonologically identical and orthographically identical words are substituted. The result is that the literal meanings are different, but the figurative meaning is the same.
Best Answer
How about to "make a mountain out of a molehill (tfd)". The link to the free dictionary says "cliché: to make a major issue out of a minor one; to exaggerate the importance of something"