Let me see if I can summarize some of your questions from my research. I'm no expert, but I keep seeing recurring themes while I'm reading and searching for data.
The magic number that lithium-ion cells like to be kept at is 71 degrees - +/- is okay but keeping them close to that is best. Much like keeping the cells charged to around 60% is best but not practical. Like Don, said extreme heat WILL damage the cells, extreme cold won't damage the cells but will reduce the effectiveness of those cells. The number at which this damge will begin is at 104 degrees. This is the absolute number I have read in many different sources including this presentation and our very own i MiEV owners manual.
Nissan had been telling its customers 140-150 degrees was the number, this is not correct as made evident by the southwest wilted Leafs. From the presentation, you can see the i-MiEV is mentioned directly in the portion about the Air method of thermal management, three types are mentioned from what looks like Good, Better and Best. The i MiEV probably has the best Air method of thermal mangement as so far no one on this forum from the southwest has mentioned battery capacity loss like the Leafs.
The Leafs simply have a fan to pull air across the cells to try and cool them, this apparantly is not adequate. From other articles I have read, Nissan rushed the Leaf to market to be the first - Carlos Ghosn the CEO of Nissan has been quoted as saying that engineers will always tell you no its not ready, so somebody has to make the decision.
Every other manufacturer has put a lot of engineering and expense into their thermal management systems. The Volt, Tesla Model S, Focus all have liquid thermal management systems which are the best but have higher cost and could potentially leak. The i MiEV does have an Active Thermal Management system using air that is managed by the climate control system. For those in colder climates, a battery warming system is available. Apparently, Mitsubishi felt this was needed enough for Canadians by making it standard equipment on those cars.
As long as the car has adequate thermal management, the battery should last ten years with 70% capacity approximately. Which isn't bad for a lithium-ion battery, try making the lithium-ion batteries in your well used tools or laptops last that long. This shows the chemistry in our EV batteries is far superior to a common laptop battery and will last if kept within its operating parameters.
For southwest EVs, definitely read the owners manual on operating your vehicle above 104F/40C. The car is designed to protect itself from damage so if driven agressively like highway operation or uphills - the power down light might come on and the car may shut down (owners manual pg. 1-10). In the future, EVs may all have liquid Thermal Management Systems or at least be offered as an option in high temperature locations like a Cold Weather package is.
http://green.autoblog.com/2010/01/25/is-the-nissan-leaf-battery-pack-under-engineered/
http://gigaom.com/cleantech/tesla-ceo-nissans-leaf-battery-is-primitive/