A lot of the crowns I do today are replacing an old metal based crown with all porcelain. The technology today is at the point that for a single crown there is no reason it can not be all porcelain. I would have them replace the crown with either emax or zirconia, They are strong, kind to the opposing tooth, and better accepted by your tissues then metal. There is always risks with any dental treatment but it is low and if it were me I would have it redone.
I am not a fan of the all on 4 restorations for the upper, but it does seem to be popular since 4 implants are cheaper then 6. You are experiencing just what most of us dentists try to avoid - not enough implant support or restorative strength to withstand the forces in your mouth. As to the broken implant screw, they are usually easy to retrieve with the right instrument. Removing an implant does destroy a lot of bone around the implant so I would question an immediate replacement. With all the complications you have been through I would recommend seeing another dentist to explore the options that would be right for you. We are all different and a cookie cutter all on 4 does not work for everyone. Best of luck.
Your pain would have to be diagnosed in a dental office. You are right that crowns do not have pores and are not themselves the problem. You could be sensitive from the cement or bonding that was used to place the crowns. One or more of the crowns may be abscessing needing a root canal. Your bite may be off to the point off causing discomfort. To answer the question, about the only thing you could do that may help would be to try one of the toothpastes used to help sensitive teeth.
The best thing to do is orthodontics. It is possible to open the spaces and create the room for implants. The orthodontist will have to look at the total facial appearance. For perfection you may be looking at more than just braces. Just from the limited information my guess is once the upper teeth are in the correct position the lowers, even if they are tilted out, will not match and your facial profile will not be ideal. I would not recommend veneers. They would be too thick and veneers would not give you anywhere near the result your looking for.
Core, or more often called post, buildups are done on teeth that have had a previous root canal treatment. A post is bonded down in the canal to strengthen a tooth that is hollow from the root canal and generally is broken down in the first place necessitating the root canal treatment. Then a buildup or core is placed around the post so that a crown can be made on the tooth. You can not see the buildup since it is under the crown. Your dentist will inform you if you need a core buildup after evaluating the tooth and how much tooth structure is remaining. Hope that helps.
