Re backup plans: Before considering backup plans it is useful to know why certain treatments fail in the first place.
Surgery fails because the PC is outside the surgical margin, is in the seminal vessicles . Most surgeons leave small amounts of prostate tissue, because they can never get 100%. The most common areas where this is left is in the APEX and around the nerves. This is the type of surgical failure than can be corrected with salvage radiation. On the flip side this type of faiure although common in surgery (21-31%) is not likely to occurr in any form of radiation.
The common cause of radiation failure is too low a dose, dead spots, and cancer outside the margin. It is also harder to kill very large tumors in a very large prostate.
The most common salvage techniques for failed radiation is brachytherapy, both low and high dose, cryosurgery and HIFU. All of these have the exact same cure rate as salvage treatment for failed surgery, because all salvage treatments are mearly cleanups for a botched primary treatment.
For low grade PC, which you have, it is rare to have positive margins or a large tumor that spreads into the nerves, seminal vessicles or nerves.
This is why all primary treatments for PC, surgery, external radiation and seeds all have the same cure rate. Radiation does a better job in getting to the hard to get at places where cancer cells may be left behind such as around the nerves Apex and 10mm into the prostate bed.
Choosing a primary treatment based on being able to use a secondary treatment is a fool's folly. Choose the best primary treatment based on the biology of your cancer and its size and location to cure it the first time.
Also with low grade PC the risks of every dying from it are very slim, so a major consideration are the side affects from the treatment option. For a younger patient the side affects should be the primary consideration because the cure rates are so high and you have many years left in which to suffer the side affects of the treatment.
Do not exclude Active survelience, because you can always be treated later with the same results as immediate treatment. This has been confirmed by many studies.