I think you should first go for root canal treatment then Invisalign and after that Teeth Whitening treatment would be better.
As I can see from your images Invisalign braces is the perfect option for you. Rest you need to visit dentist for the treatment.
Yes, you need to maintain your oral hygiene so as to prevent infection. You can use electric brush and rinse your mouth properly.
This is not a normal thing that you were facing same problem after 3 years of the treatment. It needs to properly examine your teeth to find out the problem. I would recommend you to consult some other dentist regarding for the treatment.
Thanks for asking the question. Well, that totally depends upon the tooth cavity. If the cavity is minor and veneers is not affected with it then your dentist will clean the tooth and use the older veneer otherwise you need new one after tooth filling.
