Welcome fellow mac mini man! I had many of the same concerns. I did two things that I would recommend to help shield the heat from the car in general(cause I live in S Fla.): Tint your windows and use a windshield visor!!!
1. I don't know but I wouldn't risk letting it get hot to the touch from sunlight, or operate it in an uncooled environment for too long. (like a car in the LA summer heat with no ac on)
2. Shade it well, but not with a black surface that is near it, because that would probably attract more heat than repel. The mini is white, which reflects the sun's heat.
3. Probably. The humidity there is pretty high, so you would run the risk of condensation.
4. Not so much.
5. The problem you hadnt thought of is that Cingular is my service provider and you should hook me up with a cheap phone.