So sorry to hear you had problems before, and that you understandably have worries now.
I don't think it was the anesthetic that would have caused heart problems although there is a tiny chance that there might have been some contribution in an older dog that became stressed -- have you discussed this with your vet as a cause and effect issue? in fact it might have been more likely that it was the existing dental problem or even the dental work itself and need for dental care that may have contributed to the development of heart disease (see below) but almost certainly there was already an issue there.
However, I think it more likely that the two events are just coincidental -- in other words that the fact that it was discovered that the dog had heart disease in the months following a dental workup had nothing to do with the actual dental workup. Heart disease is the single most common disease in this breed. Almost every Cavalier over age 10 will have mitral valve disease (heart disease) -- sadly, it is endemic in the breed now and half of all cavaliers will already have a murmur by just age 5. So it may well be that the murmur was only picked up at that point but it would be extraordinary if none of these dogs had heart problems before such a good age.
There are some better lines for heart health, but even within those most dogs would have a grade one or two murmur by age 9 or 10 or so; it would actually be quite rare for older dogs of that age to have none at all. Had Norah been checked by a cardiologist as opposed to a vet before the murmur was picked up? Most vets will miss early grade murmurs that a cardiologist will quickly pick up. That's why I think perhaps the murmur just wasn't heard even though it was probably already there. Indeed, because heart disease is so prevalent in this breed, the average life expectancy is only about 10 to 11 years.
That said there is a connection between gum disease and heart disease. It is possible that the bacteria from gum disease caused the worsening of heart problems that were already there after the dental work. Because of this slight risk, many vets will put dogs, especially older dogs, on a cycle of antibiotics to treat any bacterial gum disease in the weeks coming up to having dental work done. On the other hand, it could be that the very fact that the dogs needed dental work meant the bacteria had already contributed to heart disease and what you were seeing was the worsening of the heart problems that were connected with gum or teeth problems (it's the same with humans -- it is known that gum disease contributes to heart problems).
Because of the connection between the two, it is important for dogs to maintain good gum and teeth health, so this can be a bit of a chicken and egg situation. For older dogs, vets usually will weigh up the best approach. It is a good idea to have advanced blood tests done for an older dog to consider whether anesthesia is going to be okay -- generally -- it is only considered a risk if the dog already has significant heart problems although in a very small number of dogs a particular type of anesthesia may be riskier.
The best thing to do is to discuss all these issues with your vet. if you haven't had your second Cavalier checked by a cardiologist, it might be a good idea to do that especially as he too is an elderly dog. If nothing else you will have a baseline idea of where your dog stands right now in terms of heart health, which is really advisable for a cavalier of that age.
It is very painful for a dog to tolerate rotting teeth -- exactly the same as you would feel if you had to endure toothache every day. So this is something you will want to have taken care of. It might be a bit reassuring to know that this is a pretty common procedure for older dogs.