1. Yes, the game models world events in. For example, 9/11 causes a large drop in demand. There are no news items about them, but you'll notice how it fluctuates over time.
2. Route marketing basically does nothing other than waste your money. It has no impact on demand.
3. Demand is static, but if you mean your load factors for a given route for the seats you supply, then fleet type, CI, dep/arr time, length of flight, RI and frequency can impact the loads you pull down.
The demand "estimates" you see are true estimates and are not fully accurate. Often times, when you see an estimate of "100 pax" per day and you offer 150 seats, you can sometimes get full loads if all of the conditions are right. The estimates can also be on the high side...