1. 2.3 is faster then 2.2, noticed a significant improvement in games (angry birds etc.) and scrolling/zooming in and out on browsers also goes more smoothly
2. The LG updating software did mention potential harm from disconnecting phone etc. I don't know if it does tough
3. No idea
4. Speed improvement (as mentioned by 1.) was the only thing I noticed perhaps a minor battery life improvement (slightly diffrent icons aswell but thats just a matter op personal taste rather then an improvement)
5. Still had all my contacts and apps after updating to 2.3
6. There was a thread around here somewhere about downgrading a optimus one so I guess it is possible. Can't realy imagine why you'd want to tough.
After updating it took a very long time to boot the phone (even removed battery once and tried again, was panicking a bit) just so you know.