1)Why does increasing pitch decrease airspeed and why does decreasing pitch increases airspeed?
2)I understand when we are using aileron to turn,one wing lift is more than the other,hence we roll to either right/left
1) although you are correct, you also have to consider drag.
Changing pitch will affect your Cl, and your Cd (Coefficient of drag), the latter being:
D = Cd * 1/2 * ρ * V^2 * s. Notice the similariles with the lift formula.
Make sense now?
To understand more, refer to the Lift and Drag diagrams as well.
2) absolutely correct. Deflecting a aileron will change the chordline of (part of) the wing (chordline being from leading edge to trailing edge) thereby changing that parts AoA (angle of attack, alpha) and its Cl and Cd.