Okay, I will go out on a limb and suggest the following term and matching usage. The more feedback you give here will help others get you a better answer.
The setup banks on the word doublet:
In etymology, two or more words in the same language are called doublets or etymological twins (or possibly triplets, etc.) when they have the same etymological root but have entered the language through different routes.
The term false cognate:
False cognates are pairs of words in the same or different languages that are similar in form and meaning but have different roots.
Which uses cognate (and ties back to doublet):
Cognates are words that have a common etymological origin. Cognates within the same language are doublets.
Your question as I have interpreted it is asking:
What is an appropriate term for a set of words that have similar but not identical roots?
"False cognate" would work here except for the specification that this "set" needs to be conjoined into a phrase or usage such that the set could have been formed due to the false cognate. In other words:
What is an appropriate term for a phrase birthed from a false cognate?
Since your example uses two roots from the same language, we can also say this:
What is an appropriate term for a phrase birthed from a false doublet?
A false doublet in this context would mean:
A false cognate in which both roots are from the same language.
Therefore, I propose the term "idiomatic false doublet":
An idiom or set of words that use multiple roots which could easily be mistaken for each other. These roots are from the same language and often have similar spellings, meanings and pronunciations.
Comparing this term with your original example and using everything together:
"Break wind" could be an idiomatic false doublet formed from the separate roots "blewwanan" and "blāwan". The confusion and similarity beween the two roots may have resulted in the now common phrase.
The only thing missing from your request is the implication that the connection between roots is responsible for the coinage. I don't really see this implication as a useful addition to a technical term that is already assuming so much of the idiom and its roots. The discussion of roots already suggests a related etymology; I hope that is enough to satisfy.
Of note, I am completely guessing and shooting in the dark. Anyone with more knowledge of the technical terms is welcome to weigh in and contribute some accuracy or corrections. My intention was to get something in place so as to provide any answer at all. Even if it is wrong or useless, hopefully it will inspire a better response.