1, sounds the same to me?
2. depends on how you plan to round over edges. If you overhang then you can heat the plastic and round it over the metla plate. Otherwise you will be using a mechanical method like a bullnose bit on a trim router to round over the edge.
3. no tests have been done to say one is better than the other. Look at Bar’s initial videos, he uses the routers’ base which is 6" or so in diameter. Engraving warped wood? then a small sled might be better because it follows the contours/ warped areas better.
Remember the router doesn’t need to be in middle of round sled either.
4. the more the marrier. tell the laser company to keep the round center hole cut out and that way people can use it to center any router they try to make / modify to fit. having 6mm central hole will help too.