Do you have access to a cardiologist or an internist who has cardiology experience? I have a 9 year old who had not been spayed because of serious heart disease. She started going into prolonged heat cycles every 2 to 3 months, and had a vaginal infection the last time. I had her reevaluated by the cardiologist, who decided that there was more risk from a uterine infection, which would be an emergency that she probably wouldn't survive, than from spaying her under a well controlled anesthesia. The possibility of losing her scared me to death, but we went ahead with the surgery. That was over a month ago, and she's happier and healthier now than she has been for a long time. She's even started joining in play with the younger ones.
I think it might be worth checking out. Dental and gum disease are killers. They affect the heart, kidneys and every other part of the body.
Good luck, whatever you do.