Common sense suggests that fear of snakes and fear of spiders, for example, could be lifesavers. No, Thorndike never said this. Visual displays, for example, would not count as pheromones. So a pheromone is a way of communicating over a distance with members of the same species, by chemical. The response may be to congregate, to flee in alarm, or some other response.
Sexual responses are one major category of behavior which can be triggered by pheromones.
No, this is just one function served by pheromones, among many others. No, pheromones can also trigger other sorts of behavior. This distinguishes pheromones from stimuli such as visual displays, vocalizations, and touch stimuli. No, this was not the point of the Skinner and Epstein research. Through a clever shaping procedure, and by fashioning little collars for the pigeons, they demonstrated that pigeons could use their mirror images to locate and peck spots of paper on their bodies which they could not otherwise see.