Use a lightmeter to get you in the ball park. Even better, if your camera has a live histogram use that for exposures that are not using a flash gun. Be aware of how lightmeters work (ie: is it an"average" scene?) and evaluate the scene when making your decisions.
The focal length doesn't change. However the equivalent (in 35mm camera terms) field of view does change. So in your case multiply the focal length by 1.6.
As far as proper exposure goes, see this old tutorial of mine here on UHH.
From personal experience at least 10 years unless the camera has been used a real lot, like for sport shooting, and then it is likely to be mechanical failure.
Both. Often, a what comes along becomes a setup. The first image below was sorta a what comes along, the second was 100% setup - the variables were time of day, sky conditions and camera position.
Both, and it depends. Locally, especially the sunrise/sunset shoots are planned, for location and timing and weather conditions, or sometimes just going for a walk may do the trick. On vacations with family, especially overseas it is almost always unplanned and it is grab what I can. The only planning is itinerary and when to go. My photography has dropped off a lot for the last 18 months due to family events etc.