1. It's only stupid if you're a redneck and shout "HEY Y'ALL! WATCH THIS!" beforehand.
2. As long as the power outlet provides sufficient amperage, you'll be fine.
3. An ad-hoc wifi connection would work with no additional hardware. However, ad-hoc connections are a bit trickier to setup. A router would be far simpler as far as setup.
Bear in mind that you'll have to get the WiFi signal to the other car which is (hopefully!) a safe distance behind you through the sheet metal and glass and everything else of two vehicles. That will require a powerful WiFi signal and a sensitive receiver.
Your best reception would be with high-power WiFi adapters and antennas mounted outside the vehicle. This could get expensive adding $100-150 to the install.