I would also like to say that since the age of your child is just 8 years extraction will be a better option compared to root canal.
cavities are due to improper brushing habits and other reasons like infection yes root canal can be helpful for your child rest you should consult a dentist he will review the problem and suggest you a solution accordingly.
That's a good question because tooth #14 which is going to be extracted is above tooth #19. Is there a plan for replacing #14? I agree that you may want to get an Orthodontists input on this. Keep in mind though that replacing a missing tooth can be more expensive than saving the natural tooth.
Best of Luck
If you are concerned about that, it may not be a bad idea to consult an orthodontist or your dentist about that possibility. Normally they would remove another tooth (premolar) to create space as the premolar is more towards the middle and the first molar (#19) is further back.