If I understand you correctly, you are saying that the Canaanite shift is a Sprachbund phenomenon and is not due to a shift in proto-Canaanite and may therefore very well have happened much later and have remained productive for a much longer time.
Basically. But I think Sprachbund is a big word. This is essentially how most languages changes happen in languages with a large number speakers. Languages are not like species. While species can only be looked at in terms of parent-to-child, and any parallel developments in different species must be explained as coincidental or simply driven by the same factors, languages can and do all the time actually influence each other. This is how dialect continuums are formed where isoglosses are not able to separate the dialects into clades, because different isoglosses would split the dialects in different ways. These originate from changes that start off in one area and spread out from there. Different changes may start in different places, and progress in different directions.
If you want to call that a Sprachbund, then go ahead, but in that case almost everything is a Sprachbund.
But that would mean we would have to bridge a gap of at least 500, possibly closer to 1000 years between the breakup of proto-Canaanite and the first codification of the Torah.
I'm not sure why the Torah is relevant. Writing wasn't invented for the Torah. Hebrew and Canaanite writing has a much longer history than that. Plenty of time to develop spelling traditions.
Furthermore, scribal schools were not monolingual. Scribes were familiar with their local languages and with the lingua francas of the region. Often spelling conventions are transferred between the languages that the scribes use, in both directions.
3. the א is etymological spelling,
Just to reiterate, if the glottal stop was lost in the singular but not the plural, then no scribal tradition is necessary to explain the spelling, as the spelling could be by analogy to the plural.
I find that combination of the three into a single explanation a bit weak
Just wondering what makes you say that.
and the Sprachbund argument ad hoc to overcome this weakness.
If you want to call it an "argument", the "argument" is not ad hoc. It is actually not connected to the word ראש. Rather, what I mean to say is that most changes in large languages (or dialect groups) occur this way, and there is no reason to suggest that it
must have been an "inherited" change from a supposed singular proto-Canaanite language.
While a Sprachbund can be stable for a long time, sound shifts are rarely productive over an extended period of time.
Regardless of how long or short that time is, it still possible that the loss of the glottal stop in ראש took place after the start of the Canaanite shift.
For the explanation to work, you would have to assume that the standardisation of spelling,
There is no need to assume. It is clear that spelling had standards. A neat example is abbreviations that are found for measures in epigraphic Hebrew, for example ש for שקל, and ב for בת, and קמ for קמח. Such abbreviations can only exist in an environment that has a concept of conventional spellings.
I am just not yet convinced that the argument is strong enough for the degree of confidence with which you have presented it.
And how do you measure my degree of confidence? So far the most confident thing I said is that one should not assume that the Canaanite shift must have predated the divergence of Canaanite languages.
