1. Ring around... different dealers have different service costs.
2. Oils will degrade over time regardless of km travelled. It is still a good idea to get it changed. I never reach the 15,000km they expect every 12 months, but I still get my servicing done at the 12 month mark.
3. Probably the sensor under the seat, but it's a safety issue, so I'm not sure a dealer would disable it for you.