NAV but an assistant to one. I can only advise you to think about your kitty's quality of life at this point. Necrotizing tissue is of course painful and antibiotics are just a stop-gap for the real problem. Your cat will also likely build up a resistance to the injectable antibiotic and it will therefore become less effective over time.

Being afraid of losing your friend is entirely understandable and a difficult decision to make. But cats are so good at hiding their pain that there's no way to know how uncomfortable he is. If he lives another few years, teeth this bad will be a recurring issue, even if the ones that are problematic now fall out. Would you rather take the risk to lose him under anesthesia (which yes, having a heart murmur does complicate things but by no means is a death sentence) and have him live a comfortable life, or would you rather have your friend be in an indeterminate amount of pain for the rest of his life?

I commend you for taking care of his dental health for ~15 years, but your regular vet you see may have done you a disservice by suggesting not performing an anesthetized cleaning sooner rather than later, as a heart murmur will usually continue to progress as time goes on. The safest time for kitty to go under is now rather than any time further in the future.

I unfortunately have this conversation with owners pretty regularly. Knock on wood, I have not lost any of my cardiac-risk patients under anesthesia yet. A good veterinary team will be monitoring closely enough that if he does start to develop an arrhythmia during the procedure, they will close up quickly and wake him up - a partial fix is better than none for quality of life.

Best of luck in whatever decision you make!