As to when assembly was undertaken, I would suggest 'during December 1940' was the absolute earliest, with 'during and from January 1941' more likely, spreading to May 1941. Shipment would then have been undertaken in say spring 1941. Don't forget that these huge supplies coincided with the Southampton CMD having been bombed, thus requiring temporary alternative assembly facilities for the Canadian 1st and especially 2nd Division vehicles. In many respects I can see that, on reflection, shipping British-order vehicles straight off to Alexandria and perhaps even West Africa was an answer to a problem of what to do with them in the light of the German invasion apparently having been postponed.
As you say, when the AIF returned to Australia, vehicles had been purchased by payments made through the High Commission in London. It made complete sense all round to ship them via Suez for further service. Ultimately survivors wre then demobbed.
