Hi Iam Ian,
Oh my...what a question! Certainly you are fortunate that your wife bakes (when I try a cake mix, my family "casually" mentions that we do have perfectly good Oreos in the pantry

) and of course you are both fortunate to have an Insight.
If you run the AC in econ it should autostop which would help FE once you are on the road. Rather than pre-chilling the car, maybe you could partially freeze the frosted cake beforehand. Does sunlight fade frosting?! This is way beyond my meager baking experience! Presumably putting the cake in a box would keep sunlight off . The storage well in the back has an insulated cover that might hold a medium cake.
There are many posts regarding window tinting. Try the forum Search. I'm currently looking at the Llumar Platinum Plus 35 and the FormulaOne Premier series window tints
http://www.insightcentral.net/forum/vie ... php?t=2781
In general the car should be "batter off" as far as heat reduction, with a good quality tint. I believe the primary reason is to protect the battery pack, but no reason it could not also protect your cakes