All the symptoms you describe sound to me like expected side effects of seed brachytherapy. That's not to say they are common, but they are a known possibility.
One thing to understand is that the brachytherapy treatment starts when the seeds are inserted. It continues for around a year, but with rapidly decreasing effect, so half the treatment is in the first 3 months which is when the radiation is most powerful, and when your symptoms started, a quarter in the next 3 months, an eighth in the next 3 months, a sixteenth in the next 3 months, at which point it has become insignificant. The timing of symptoms seem to me to fit perfectly. At this point a year after treatment, the body should be recovering from the side effects, which is what you're seeing. There is always the possibility that some side effects might not go completely and incontinence is one of those, but it's too soon to assume that yet - I would expect there to still be improvement. As with any radiotherapy, some other long term side effects can appear later.