This does not weaken the argument, because the astronomer never contests the point that Mount Shalko is a unique ecological site. Instead, the argument is that the observatory would actually protect the mountain’s ecology by precluding recreational use.

This does not weaken the argument, because the number or type of people who oppose building the observatory has no bearing on the astronomer’s argument. Just because there’s broad opposition, that tells us nothing about the merits of the astronomer’s claims.

This weakens the argument by undermining the astronomer’s claim that the observatory would protect the mountain’s ecology by eliminating recreational use. If the observatory could be just as harmful, then there’s no ecological benefit to building it.

This does not weaken the argument, because it partially affirms the astronomer’s assumption that no new cities will soon develop near Mount Shalko. In fact, by doing so, this strengthens the argument.

This does not weaken the argument because whether or not other nearby mountains are suitable for recreational use is irrelevant. Mount Shalko is already used for recreation, and the observatory would stop that use. This doesn’t change any of that.