Depends how much it affects you and do you have the money for extensive surgery. Thai oral surgeons are very good most have all the latest technology for very good reconstruction. Without a retainer though your teeth will be on the move.
A root canal is a last resort for pain management. Sounds like you are getting close to needing one. Either you pull it or root canal depends if you have the money to plug the gap. Root canals not all bad can be trouble free for decades but yes it's like a dead tree stump left behind.
Home bleaching is pretty useless and it certainly won't turn a decayed tooth to a shiny new white one. If you teeth are pretty horrid yellow ask for in house bleaching first then Color match to a composite to repair the bad tooth.
Ask to have your lower fronts bonded in a bridge to keep them fixed in place using ribbond and composite. It's a cheap quick procedure and can last for many years sometimes to stop those wobbly teeth falling out.
If there is no pain and not unsightly you can have any decayed margins resealed with flowable composite then it should last for many more years. Sometimes if it "ain't broke leave it alone" is well justified.
You shouldn't be poking your tongue in the holes makes things worse. Get hold of some MMS chlorine dioxide and use twice a day as a mouth wash. I presume if the dentist already seen you twice they are not overly concerned and you just need healing time with pain killers.
Unfortunately no matter how bad your teeth are now they just get worse without help. Go have a chat to a dentist and he will explain what can be done and the possible procedures then give you time to think things over. Please do it as infections can be dire! Most pain of dentist is in the mind...
A lot less tooth material needs to come off with veneers and crowns need basically ground down to a tooth stump. Why not consider direct composite make over its a lot cheaper and faster it will hide the discoloration.