Bleaching treatments can hurt your gums if the active whitening ingredient is in contact with the soft tissue for a prolonged period of time. Proper fitting bleaching appliances should not allow the active ingredient to be in contact with the soft tissue. That is one major reason to have whitening done with professional dental supervision. Touch up treatment frequency depends on lifestyle and can be done safely at home with proper instruction.
The treatment of choice to restore the color of your natural teeth is bleaching. Their are several products that can accomplish this. In office bleaching is probably the most time efficient, convenient solution. A take home system is the most cost effective. After bleaching, if you are still un satisfied with the color of your teeth, a more aggressive solution such as porcelain veneers should be investigated.
The cost of the implant and bridge are going to be about the same. The biggest factor in your decision should be what is the status of the adjacent teeth. If they have been restored then a bridge makes perfect sense since the integrity of the tooth has been compromised already. If the teeth are virgin teeth, than an implant makes sense so that you don't have to invlove the perfectly healthy adjacent teeth in restoring the lost tooth. Both restorations will last a very long time if they are done clinically well and cared for properly. If timing is an issue, the bridge is completed in 2 weeks, the implant is going to be a 4 -6 month treatment.
Smoking stains are more difficult to remove from implants than regular teeth because you can't use an ultrasonic cleaner to remove the stain. However, they can be removed with a fine diamond polishing paste that will not scratch the porcelain. It takes more time and a bit more effort but can be done effectively with a good dental hygienist.
The most predictable solution for your case would be extraction of third molar, upright the second molar with orthodontics and then place a dental implant to be restored with a fixed crown. If the time commitment for orthodontic movement and implant placement are a concern, you could consider extracting third molar and fabricating a three unit bridge to replace the missing molar. If the second molar has not drifted to far forward, it could be uprighted in the preparation design for the bridge and this treatment could be completed in 2 -4 weeks.
