Crowns don't often change color so it would be good to know the cause of the discoloration as that can be indicative of a definite need to redo crown. Is it more in the root or gums at the edge of the crown? Have you done recent bleaching where your other teeth look whiter making the crown appear to be now darker than it used to? If you have dental insurance and want to make sure they will pay their portion they likely won't if the crown is less than five years old and you've been on that same insurance plan for the past five years.
First off, Kor keeps their product refrigerated from processing, through shipping and they make sure your dentist office does as well. This keeps the chemicals at a much higher effective capacity than others that are not shipped refrigerated. Second, there are a few steps taken in the making of the trays that are much better than most other offices do. These steps help keep more of the bleach on your teeth and more importantly it keeps the saliva from deactivating your bleach. A little bit of saliva can quickly deactivate all the bleach in your trays if they don't keep it out. Finally, the products do make a slight difference. A desensitizer is included in every kit. Most offices don't include that with their bleach. Because the bleach is always refridgerated its chemical makeup can be slightly different from most other bleaches reducing its sensitivity. In the end, you could get many if not all of these things from another system, but in my experience trying several no other product gave as much attention to detail and therefore the results weren't quite as good.
I would start with straightening your teeth first. From this picture is looks like you are fairly close to showing an ideal amount of teeth for an attractive smile. If they were straighter that might change your mind on feeling like not enough teeth show. Botox would do the opposite in that it would relax your facial muscles and make your lip hang down lower, covering more teeth. Veneers could make your teeth look longer (if that is what you mean by making smile go higher), but to keep from breaking them off you would have to also lengthen all your back teeth as well. Surgery could shorten your lip length and make more of your teeth show as well, but is obviously a lot more traumatic and costly.
There aren't any whitening methods that would be contra-indicated for someone wearing fixed retainers. The great thing about fixed retainers is that you can't lose them. The negative is that they can be harder to clean. Because of that make sure that you get a good cleaning before doing whitening. The tarter buildup can keep whitening material from reaching as much of your teeth resulting in yellow sides and white fronts of the teeth. Enjoy you new whiter and straighter smile!
There are several things that could cause your teeth to not show during normal conversation or when your lips are at rest. With your history of being edge to edge for a few years it is very likely that it is at least party due to excessive wear. It can also be due to the position of your teeth and bone relative to your lip or even your lip being too low relative to your nose. Depending on which cause(s) this, your treatment could include restorations longer than what you now have, orthodontics, surgery or any combination of those treatments.
