You can feel 'guilty about' doing something, but you are 'guilty of' a crime.
If you are guilty, you did the thing you are accused of doing. If you are not guilty, you did not do the thing you are accused of doing.
http://chalktales.blogspot.com/2008/03/pieces-from-past-part-two.html
Whichever court you are being tried in determines your sentence.
Yes, you can.
Up to a certain point, yes. It is not helpful to your defense, though, but if you can successfully defend yourself then it is worth the time.