To Montana Skier,
When lines are long, I feel that helping keep chairs at maximum capacity is polite. Whether you are waiting in the singles line or off to the side, why should anyone care. I would rather see you occupy an empty seat than ride up by yourself. When lift attendants are not helping, be assertive, speak up and ask if you may share or presumptively slide into an empty slot.
Depending upon where we are, our sense of what is proper may be perceived as odd. The ideas of propriety and personal space vary widely. While the English are used to queuing, some Europeans adopt a different style where skiers converge like cattle toward the gate while pushing and standing atop of your skis. I remember some guy asking me to move aside so that he could squeeze ahead. I think he might have perceived me as less than an alpha dog had I moved. While it seemed bizarre, they likely are equally amused by our behavior.
It is nice to hear your concern about maintaining etiquette in the sport.