Retreatment of failed rct started..on keflex 500mg 4x daily x 20 days. infection in jawbone below tooth...how likely for success resolution?

Hard to give %: A failing root canal tx can be successfully retreated. Sometimes, it also takes a surgical procedure called an apicoectomy, which is performed to clean out the bone and seal the canal(s) from the outside-in. Even after all of this, the tooth can still fail. All the best...

Better options?: There is no specific number or even range of success that we can place on your specific situation, especially without seeing you or an x-ray of the tooth. I'm not sure if "infection in jawbone below tooth" was original problem or still now after 20 days of keflex. If the latter, the chance of success is unlikely. Consider an apicoectomy or extraction. It's a prof judgment call.

Depends: May need another type of antibiotic, like amoxicillin. If the infection does not resolve in approx. 10 days, the tooth may be cracked and extraction may be the only option. An apicoectomy may work, where the dentist does a minor surgical procedure in the tissue/bone below the roots to get to the infected tissue that way. Implants, paritals or a bridge may be indicated if the tooth is taken out.