I don't use a pre-breeding feed for my adults but I always give them a quality feed year around. It is possible for poor nutrition in the adults to cause poor hatches or problems with newly hatched poults. Poor genetics can also cause problems. Leg issues can be nutrition caused, genetics caused or even incubation problems.
Your humidity issue may or may not have caused a problem although high humidity tends to cause more problems than low humidity. Since you don't normally candle often enough to verify the proper increase in air cell size, there is no way to tell if the humidity was an actual issue. I tend to think it may have had an insignificant effect.
One possible issue is that temperatures that the eggs have been in can adversely affect a hatch. My research shows that 27°F will kill the embryo. Any eggs that have been exposed to temperatures below 27°F should not even be set.
I do tend to bring in new bloodlines from time to time to avoid too much inbreeding. I try to avoid the brother/sister breeding but do use linebreeding.
