Thank you, Quiviscumque, for your reference to the
scholarly article by Volker Noll. Now we're getting somewhere!
Noll refers to "the Berber thesis" of Elcock (1960), which I understand as follows.
Many, perhaps most, of the "Moors" in Spain spoke an Arabic that was heavily influenced by a Berber language (either their first language, or that of their immediate forebears).
Berber (says the theory) has no definite article, so these Moors, in speaking Arabic, attached the article to the noun indiscriminately,
regardless of the noun's grammatical status as definite or indefinite.
Accepting the claim that Berber doesn't have articles, I find it much more plausible that the obscuring of the article-function of "al-" was done by Berber-influenced Arabic-speakers, rather than by Mozarabs, who had the article "el" in their Romance speech.
So maybe the Mozarabs didn't have much opportunity to hear the Arabic nouns without the article, and so maybe for them the function of "al-" as an article was muffled.
But Noll points out some problems with the Berber thesis:
less-frequent preservation of "al-" in the Arabisms of Sicily and Valencia, Arabic loans with and without the article in (modern-day?) Berber,
and chronological questions that would take up too much space here (the button says "Post
Quick Reply"!).
Noll ends up rejecting the Berber thesis, and seems to prefer instead an explanation based just on the greater frequency of with-article use over without-article use in Arabic generally.
Turning to written language (Arabisms imported through translations), Noll points out the non-separation (not even by a hyphen) of the article from its noun in Arabic script.
(And, getting off the track here, forms like the star-name "Aldebarán"—with the "l" not assimilated to the sun-letter "d"—show that they were borrowed first in writing, not in speech.)
Noll sees some significance to the Andalusian Arabic practice of preserving (rather than eliding, as in Classical Arabic) the vowel [a] of the article.
But I don't see what this contributes to answering the question of why "el al...".
The two articles are similar enough in form that I am more persuaded by those arguments that say
the awareness of article-function for "al-" was lost somewhere along the line of loanword transmission.
I still can't imagine that happening in Mozarabic, so—in spite of Noll's paper—I think I favor the Berber thesis for now.