do we know it wasnt pronounced cangam then? I know it sounds odd today
but bear with me.
We know tolkAppiyam got into phonetics, and precisly on this related topic. But it looks like it did not lay things unambiguosly, since if it did, there wouldnt even be a possibility of a discussion right?
Btw, i also think it was sa then. But i would like to see some evidence to remove the ambiguity. IMO, one cannot ignore malayalam that easily since ca at beginning is not a sanskrit influence as sa in beginning is very common in sanskrit too. If I remember jayaram right, there are common words between tamil and malayalam, where malayalam uses ca. But again, in spite of all this my leaning is also towards sa in the beginning (in tamizh) from olden times.
(correction added later
: Although sangam - doesnt that have the same root as sanskrit word for gathering? So in this case it would have been sangam. Of course then you get into the rathole of which language had it first