I'm an ATC with about 500hr in light singles. To try and answer your q's:
1)From memory, the definitions go...Light; seatbelts not required,lulls pax to sleep, Medium; Seatbelts required,small objects move around cabin, Severe; Seatbelts required, aircraft is very difficult to control and will be out of control at times, structural damage may occur, Extreme; aircraft movement is violent and is out of control most of the time, structural damage occurs. (In practice, severe is rare and extreme almost unheard of.)
2)Weather forecasts-often based on pilot reports; cloud observation,eg: the rotor mentioned earlier,or seeing a thunderstorm visually or on the radar; situational awareness eg: wind flowing over hills will produce turbulence-the stronger the wind and the bigger the mountains and the more inherently unstable the atmosphere on the day the rockier the ride; and sometimes it just comes out of the blue. (Which is why it's a good idea to keep the belt on even when it's smooth.)
3) Personally, slow to turbulence speed or Va, try not to overcontrol, make a joke about counting the wings to reassure the passengers, and depending on the cause, a change of altitude or route can help. As an airline passenger, I keep the belt on whenever seated and marvel at the flexing of the wing.
4) Yes. But this would be rare. And I think for a large aircraft, very rare.
5) Sometimes a little. As a passenger in an airliner, very seldom. Don't think I've ever experienced severe turbulence in an airliner. Maybe moderate to severe.
6) In the word of E.K.Gann,( "Fate is the hunter"), "It's margins of strength are beyond my understanding."
If I remember rightly, lighties are certified to +3.8 and minus 1.7G. Air transport aircraft, such as a Boeing, probably similar or greater. There is a 50% margin required on top of that. You hardly ever hear of inflight breakup as a direct result of turbulence.
The more likely hazard is windshear related-a type of turbulence that results in a loss of airspeed with a change of altitude. This can be hazardous on final approach. At the airport where I work, it's a frequent occurrence. Seen some interesting approaches; never seen a crash.
7) When on my first solo cross country in a hilly area.(Central Otago, NZ. Famous for gliding.) Flew just underneath a rotor cloud. 'Course I didn't then know it was a rotor cloud, it was smooth as until then. Stuff hit the ceiling, including my head. The prop made interesting and discordant noises. Roll reached about 70degrees.The VSI went off the stops,twice. I was quite unhappy.
I would recommend reading everything you can get your hands on about Met. Some people find it a bit boring or difficult. I think it's one of the most fascinating subjects of the natural world, and can often easilly be related to what you're seeing from an aeroplane, or indeed from the ground. Good luck, take charge, live long and prosper.