1. Differential is used to correct adverse yaw (the down aileron creates more drag, yawing the plane away from the bank) and make more axial rolls.

2. You have basically changed the servo center point. You probably have the same travel up and down from the new centerpoint, unless you're hitting the servo endpoints.

3. To use expo, or how much to use, is a personal preference based on how you like a plane to feel and respond. A lot of people that learned to fly before expo began being used probably got used to the feel of linear response, and don't want to change. Personally, I use expo nearly all the time to differing degrees based on throw, airplane, setup, CG, etc. Just adjust it 'till you feel comfortable with it, wherever that may be. Be carefeul not to put too much expo on a surface, because then you will have nearly no response around neutral so you keep moving the stick until, WHAM, you get a lot of response - maybe too much.

Hope this helps.