Porcelain veneers can help you re-establish the length that you desire but it will be limited depending on your bite. The aesthetics of a new length can be tested out by placing a resin bond or a temporary which can be fabricated by the lab. From your photos 3mm seem to be too long. But the most important thing before you commit to any veneer treatment would be to determine why one of your canines chipped and what can be done to prevent this from happening again? Otherwise this could reoccur with the veneer as well.
The best way to prevent bone loss around dental implants is to maintain proper dental hygiene which would include flossing. This would help prevent gum disease which will lead to bone loss in implants in the same way it would with natural teeth. Regular periodontal maintenance (cleanings) and checkups by your dentist are a must. Another factor to bone loss could be a none ideal bite which generally causes grinding (bruxism).
Porcelain veneers are very easy to maintain as long as you maintain them the same way you would your other teeth. This includes proper flossing, brushing and regular dental followups with your dentist. If a problem was to occur it will most likely be caused by cavities, gum disease and a none ideal bite which usually involves grinding. What you do need to keep in mind is that even if you do everything right there is always a small risk that a problem could arise the same way a perfectly healthy tooth might fracture/crack. If you are concerned about stains than you have less to worry when compared to your natural teeth/enamel. Stains can very easily be polished off porcelain veneers while the removal of certain stains (i.e. red wine, curry etc...) are harder to remove from the enamel especially if they have been there for a long time.
Generally minimal removal of the enamel (Enameloplasty) is safe but if already too much of your enamel is missing maximum care should be taken to prevent further loss. The enamel is a very strong material that protects the teeth and once it is lost the tooth becomes more vulnerable to cavities, faster wear of the teeth and potential loss in the vertical bite dimension. Should a tooth ever require a dental restoration you can always achieve a much stronger bond and seal between the enamel and the restoration. There are times when enameloplasty is required to re-contour the shape of your teeth to achieve a more favourable appearance or to correct a bite interference.
The most common cause of loose teeth is due to bone loss which is most commonly caused by periodontal disease. Depending on the extent of the bone lost such mobility can be reversed with proper periodontal therapy which might include root planing and laser treatment. In certain situation regeneration of bone may occur. The sooner you address such problems the more favourable result you will be able to achieve.
