The glass should be blocking out almost 100% of the UV radiation from the sun. You will not get burned if the windows are up. Richard Feynman watched the whole trinity test through the window of a truck without any other protection.
I'd imagine tint would only really help if you have a dark interior, but I suppose it won't hurt. I would go one level over legal on the sides and the next level above that on the rear. If you see a cop, roll down your windows.
The transitions lenses sounds like a creative idea, but I wonder what it would cost (if the coating was available)? The welder's helmet idea wouldn't work over so well simply because it requires a current to stay clear. The tint comes from a LCD panel, I believe.
Unless you drive for long periods of time, I'd rather invest in a sunshade or a white car cover for the times you are parked outside.
- LostCause