This strengthens the argument. It rules out the possibility that environmentalists on television raise scientific issues that, like social issues, have important political consequences.

The number of opposing views is not relevant to the argument. Advocates of multiple positions can be given equal time even if there are three or more positions.

This could refer to only a small number of social issues, so it doesn’t refute the author’s general assertion about them: they can “seldom” be settled definitively.

This ruins the author’s basis for contrasting social and scientific issues. If scientific issues are like social issues in this way, then by the author’s argument programs should offer equal time to opposing views on scientific issues as well.

This implies some networks may be reluctant to discuss social issues, not that those networks would give equal time to opposing views on scientific issues. It’s possible the same networks would refuse to broadcast programs discussing scientific issues.