The only one of your questions I can truly speak to is your second one. A 15% or 20% film applied over the factory privacy glass(almost always meters dead on 20% in my experience) in that application is going to net about 5%. Whether or not that's "too dark" is largely dependent on the individual making that judgment for themselves, if you're hoping to maintain decent visibility out of the rear windows at night I'd recommend going over them with a lighter film. A 35% will typically net in the 8-10% range and a 50% will get you closer to 15%. As far as your number 3 concern, I haven't done any full windshields but I've done plenty of sun strips on them and I've only ever had one that acted up and it was an older model that was fine after it dried out for a couple hours. That was also before I really started taking precautions to deal with the excess application solution.
