How to read the pressure diagram of an airfoil?
The drawing in your answer shows the contour of the tips of the local pressure vectors when they are plotted perpendicularly to the local airfoil contour. Yes, this will change with angle of attack. In potential flow theory, the local pressure can be calculated as the linear superposition of a contribution from camber and one from angle of attack. While the camber-related part is constant, the angle-of-attack related part varies linearly with this parameter.
Yes, since the diagram shows the pressure distribution at high angle of attack, the top surface shows mostly suction (= local pressure is smaller than ambient pressure) and all of the bottom surface pressure that is higher than ambient pressure. Static pressure is equivalent to potential energy and total energy is constant, so this means that flow speed is higher than the free flow speed over the red region and lower over the blue region.
Yes, measuring the perpendicular distance between the airfoil surface and the contour line will give you a pressure difference to the ambient pressure, but there is no scale, so all you can do is read relative pressures from that drawing. I recommend to use a different way of plotting pressure, see here for more.
Yes.
P2 has no specific name; here the flow accelerates through the free stream speed. The stagnation point is below the highest distance between airfoil and pressure contour near the nose (in the blue region). That the local pressure at the trailing edge is equal to free stream pressure is a coincidence - at lower angles of attack, the flow will decelerate below the free stream value near the trailing edge, so the blue region would wrap around the trailing edge. Here, flow separation at the trailing edge is imminent, and if the angle of attack is increased a bit, the pressure will turn to suction at the trailing edge.
I think this graph is not quite realistic but is meant to show the general tendency only. More can be seen when inviscid and viscous pressure are plotted together over chord in a cartesian coordinate system.