Porcelain veneers are indeed the best way to get a uniform shade on every tooth that will never require whitening. The uneven gum tissue above the darkened tooth should also be resculpted using a soft tissue laser to create a more uniform round arch to match its neighbor. Ten veneers on the upper and ten on the lower would cover the whole smile and provide the most ideal uniformity.
Another option would be to first have professional whitening and do anywhere from 2-10 veneers on the upper. The only drawback would be that you’re limited in how bright of a shade you can select based on how white the natural teeth can get. Doing 10 on the top and just whitening the lowers does give you a bit more freedom to go brighter with the uppers as a whole.
If you’re looking for a conservative way to reshape the teeth for better symmetry and proportion, I’d first consider recommending gingivectomy, which is laser trimming of the excess gummy tissue above the teeth that look short. There’s only so far you can go with this and measuring the tissue attachment will help the dentist or periodontist determine the precise amount that can be removed without creating chronic inflammation.
Enamelplasty is a fancy word for re-sculpting the shapes of the teeth using a series of polishing discs. I would recommend this on the right canine especially since it reaches lower than the left when looking at the arc of the lower lip. Again, you can only go so far with this, but it could provide a nice improvement without getting too invasive. As for the crowded two front teeth, using Invisalign express or a product like that could correct the crowding in a few months. You could ask to have a ‘fixed’ retainer to provide better retention and stability. This is a wire that’s cemented behind the two front teeth.
When administered properly, Botox is a very conservative way to address decreasing the gummy show. Referring to your photos, I’d recommend injections not only in the LLSAN muscles, but also the Levator Labii Superioris, and Zygomaticus Minor, due to the appearance that your gummy show extends from the front of your smile posteriorly to the back as well.
To answer your question, these injections would only affect the muscles that elevate the lip and should only affect your smile in a positive way (that is, covering the excess gums). It should have no effect on limiting how wide you can smile.
The photos show the narrowness of your arch from the front view quite well. To answer your question, there are several options. Further orthodontic arch development can still be achieved with an LVI dentist/orthodontist trained in physiologic orthodontics or at Dr. Gallela’s Facial Beauty Institute. There are techniques and appliances that these practitioners can apply to get that robust arch form you are looking for. It would, however, take about 2 years in appliances/braces.
From a porcelain standpoint, though it would be more ideal and complete to have 10 units of porcelain perfectly matching with optimal widths, it could be done in less. I think you’d fall short with 4. Including the canines also (making 6) would go a long way as this is THE transition tooth between the front teeth to the back and it sets the tone for the angle and thickness that you desire on the back teeth to fill the space known as the buccal corridor.
As for no prep veneers, maybe. Most of your goal is additive in nature which makes this a possibility. Sometimes very minimal prep can be more effective and dependable in this type of instance if you are open to that. That decision would involve looking at stone models and more photos to make the call.