as long as teh coco hasn't broken down, it'll continue to work well. i think "they" say 2 years, possibly 3? may find some say 1 year etc etc... likely depends greatly on the quality you start with -- if mostly pith (the dust), it is already broken down alot.
once it breaks down too much, it won't provide nutrients in a competent way for your plant and whatever other issues.. maybe compaction? lack of air? etc.
i use my re-used coco for donor plants. and haven't had a problem yet.
as far as your fertilizer, as long as you didn't overfeed at end... if you flushed for just a day or 2, it's likely got barely anything left in it. if worried, give it a good run through with 100% runoff or more... whatever is in there is best to dilute and hit the reset button, if you want to be certain. so, that just depends on what you did at end with it, and common sense from there.
if you mix after just 1 grow, i don't foresee any issues. continuing to do so over time will eventually cause an issue.
this stuff is $12 per 2 cu. ft. just buy new stuff. the effort of knocking it out of roots, picking roots out and worrying about if it is infested with bad microbes isn't worth the pennies you save. even if you don't normally have problems doing it, eventually you will cause one given enough time and a problem that would be 100% avoidable.