I often get pressure in my knee, and it's relieved by twisting my leg until it cracks. Is this safe?

Probably. These types of symptoms are usually caused by patellofemoral catching. The underneath surface of the knee cap experiences friction in the groove it sits in. This can just be increased friction between the cartilage surface, or occur because of irregularities or potholes in the cartilage surfaces. If it isn't painful it probably doesn't need any treatment except strengthening exercises.
It depends. If the popping is coming from the kneecap joint, it may be normal. However, if it is coming from internal derangement within the knee, then it may need to be addressed. See an orthopedic specialist, who can sort this out.