Given a final consonant deletion followed by a final schwa deletion, and we get the synchronic system. There are two issues here. Furthermore, the existence of words with stressed initial a which do not undergo the rule also suggests that perhaps stress is not the relevant condition. Such speakers may, therefore, not have any attested feminines with unstressed a that take el. I am not sure what the answer is, but I AM sure that that is the question.
Dialects can, of course, differ in what is possible, but that needs to be shown.
I would question this, because I do not know of any relevant published research.