Starting with the Elise provided a massive benefit: the ability to iterate. Big Design Up Front would have massively failed -- there were way too many unknown unknowns.

In the end the product was nothing like the Elise. But intermediate products were like the Elise, and could be driven and test manufactured and could inform revisions. A half complete scratch design could not have been.

Questionable. Going to a company that had experience with car body designs and getting an in-house designer would likely have been a better plan for them.

Many of their problem was due to the assumption that the electric car motor & batters from AC Propulsion were working as advertised and ready for mass production. That assumption was wrong. So the iteration was because changes in the propulsion system resulted in changes to the car, and changes in the car led to changes in the propulsion system.

Elon also claims that he was the sole founder of Tesla...

... after he bought out the founder(s).