You have to enable transparency on the plane to make the alpha channel have an effect. Try Object3D.setTransparency(0...5) and see if that helps. If the alpha channel doesn't look good, make sure that you've created the texture using the appropriate constructor or otherwise, jPCT will create an alpha channel itself by evaluating black areas in the texture.
About the faces...by default, jPCT does backface culling, i.e. only the front face of a polygon is visible, not the back face. You can disable this by using setCulling(false); However, this doesn't affect vertex normals, so the lighting of the, now visible, backfaces might not look as expected. In that case, you have to use an inverted or flipped version of the plane with culling disabled.