Doesn't matter what kit or method you use. Once the lights have yellowed and been sanded the UV protection is gone and the polycarbonate will oxidize and yellow again in far less time than they originally did. You have to clear over them with 2-part clear coat if you want them to last.
Not quite. This is marketing to extract more money, then people just follow it without assessing the long term difference.
Clear coat just adds expense and time, then time again later to strip it all off to refinish them again. It's quicker, easier, and cheaper in the long run to only put a clear wax on type polymer sealant on.
If you only put a polishing/cleaning type sealant on, you may never again have to do more than that. Worst case you can just put a dab of toothpaste on a rag to clarify them when you wash the vehicle.
This whole market for "kits" is mostly for people who have no refinishing supplies. If you have wet sandpaper, or rubbing compound, whatever it takes for the initial cut depending on how rough it is, then successively finer grit in stages, just about anything will work. Believe it or not they even make goop called plastic polish. I know, it's hard to believe.
New lenses are an option for people who demand perfection, but could be out of place if the rest of the vehicle is not similarly perfect, and for some vehicles are a lot of work for minimal gain over a properly restored factory lens. Do make sure you aim them properly.