Brad, the "experts" have answered your questions better than I could. Two quick comments - waxes and sealants are easy to confuse but not so easy to differentiate. Wax (ours at least) contain carnauba and have a natural aspect. Opti-Seal is all synthetic material. There is a difference in the look (warm, wet look vs hard shell some say), so personal preference matters. And there's a difference in durability, with sealants lasting about twice as long. Second, Optimum has been asked for years about topping Coatings (Pro or retail) and after telling people it wasn't necessary, we finally gave in and told customers they can use any wax/sealant/instant detailer that doesn't contain abrasives. My car was Coated by Yvan for a video, but I use Opti-Seal as a drying aid almost every wash. Whether it's psychological or obsessive, I know it won't last but it's part of my routine.
I get to tell new users what products to use and how to use them, and know better than most how close products can be. There are some definite differences in tasks performed and categories filled. Dr G is not into product development just to sell more, but wants to provide a compete line and really tries to improve each segment vs present a "me too" product. For example, Optimum Instant Detailer was something I never used or felt I needed until I got hard water spots on a road trip and found how well it cleaned them up. Then I used it on a car soaked in a storm and found that it cleaned well, left great (if temporary) gloss without marring, and I keep a bottle in the car for spot usage on bugs, bird bombs, etc.