alpha-blend I would guess in opengl alpha-test is used for the 1bit stuff, and blend is used for blending (decal,additive... whatever) of a surface on top of the framebuffer. alphatest is fast because it can replace the pixel below, whilst alphablend will need the previous drawn pixel and mix it with the new one
no you disable z-writing (hence intersections create those z-fight like stuff), but you definetely keep z-testing and want that a glass behind a wall is not rendered... more on those z-fight like things, when sorting triangles, it can happen that they intersect in such a fashion that they are partially in front of each…