Following other interaction with Sharmaji, one of our scholars, I asked: "Ganesh is worshipped as the first god in all yagyas. I have been trying to find out: Since when this is the case? And why? He is not mentioned in the Rg Veda."
"Lord Ganesh is mentioned in Ramayan and Mahabharat. Rg Ved mantras are in praise of nature and deities that personify its different aspects. Later, priests used these deities for sacerdotal puposes and[i] devotees to begin their puja etc. Ganesh is Ganpati, the people's god for beginnings and ceremonies. [ii] My conjecture is that he became popular in epical eras. Ganesh is more for the denizens of Sansar; Indra and definitely Brahma, Vishnu, Mahesh, Mata Durga and other goddesses belong to the higher or other Loks, ethereal regions of devatas and rakshasa like bhasmasurs etc. Parvati is often shown to intervene on behalf of ordinary people in folklore and Ganesh is beloved son of Parvati. For him, going round his parents, Parvati-Shiva as the legend says,is the whole universe."
This response is fine as far as it goes, but I think you will agree that it does not really answer my question.
So I now open the question to all my readers, and will be grateful for any light that anyone can throw on it.