Yes even if you have the genetic potential for high intelligence environment can destroy it. What counts as a good environment is pretty basic though - the vast majority of people grow up in an environment which won't kill their genetic potential.

What I wrote about the means is correct. Over the population the mean intelligence of the offspring is equal to the mean intelligence of the parents. Of course with individual parents and offspring you can have sets of children where the means are not the same, but averaged over a large group the means are the same.