Your status as a passenger may be limited, so they might cancel any upcoming trips if they suspect you of skiplegging. It’s best used if you have no additional travel with that airline planned.
If you bought a round-trip ticket, but only used the first leg on the departure, the airline would cancel your ticket and you wouldn’t have any choice but to purchase a one-way fare for the return.
I think that’s the question you asked.
That depends. If you book a hotel for a cheap rate that is dependent on the length of stay and you tell the front desk at check-in or during your stay that you’ll be leaving early, the rate will change when they change the departure date. Come up and say “we’ve decided to leave early so we’re checking out now” you won’t - in my experience - have to pay the difference. Of course, if it’s a pre-paid odds are good you’ve lost the remainder.
You don’t have to tell the front desk. The hotel doesn’t really care, so long as you paid. And I’m comparing to a pre-paid situation. No one is asking the airline for a discount because they got off the plane before the final leg. They are just asking not to be penalized for failing to get on that final leg.