I have bone loss under tooth number19. Will a root canal and/or a bone graft at tooth number18 save the tooth?

Tooth #19. This is impossible to say without doing an examination and seeing x-rays. This is best answered by seeing an endodontist (root canal specialist) and a periodontist for an examination and evaluation.
And the answer is- It's impossible for us to answer these kind of questions over the internet with any degree of accuracy. It requires the combined input of an endodontist, periodontist ; restorative dentist. You will need to consult all three and pick well respected practitioners in order to discuss your options, pros and cons of each and especially long term prognosis. Then you can make an intelligent decision.
Is the bone. Loss also on tooth number 18. The information provided in your question is limited. Best to be evaluated by your dentist for an exam and xray to determine etiology and appropriate treatment. There are no generalizations - each individual is unique. Hope this helps.
Not likely . There are a lot of factors that influence the answer. Most commonly a bone graft or root canal on one tooth (18) won't cause another tooth (19) to be necessarily saved if it is already indicated to be extracted or lost. Unless perhaps the problem on 18 is spreading to 19. A diagnosis and etiology are needed for both teeth to provide a more informed answer. Get a second opinion if in doubt.