To answer the last two posts:
Car hire location on the French side is closer than on the Swiss side. Overall timing will now swing in favour of renting on the Swiss side.

You can either drive from airport through geneva city to French motorways, or buy a Swiss carnet at border control on exiting the airport. Using the Swiss motorway saves about 30-45 mins, so that's what I did, and kindly left the carnet attached to the windscreen.