That mounting behavior from the rooster could be a sign of dominance more than sexual maturity. I've seen a dominant hen mount another hen and touch vents when there was not a mature rooster in the flock. Sometimes roosters will mount another rooster to show dominance. When I separate a rooster from the flock for a few minutes then put him back with his flock, the first thing he normally does is mount a hen to demonstrate his dominance. When they are first let out in the morning, the rooster usually wants to immediately mount a hen to show he is still the dominant chicken in the flock.
I don't put much faith in the pullet's behavior either. I've seen a pullet willingly squat for a rooster five weeks before she started to lay. I see pullets and hens try to avoid the rooster's advances even if they are laying. A pullet willingly squatting for a rooster may be an indication they might be getting ready to lay, but it could also just be a sign of submission to his dominance.
I don't know about Guineas, but I've had hatchery Black Australorps start laying around 20 weeks. Usually they are a little bit older when they start. I know you are anxious for the eggs, but I think they are better off if they don't start laying real early. It gives their body a chance to mature more.
Good luck! At 18 weeks, it should not be long. It will just seem that way.