Looking for help improving/correcting transparency and depth in a 3 pass shader.

Code (CSharp):

1. Shader "Custom/maskedTransparentDiffuse"
2. {
3.     Properties
4.     {
5.         _MaskColour("Mask Colour", Color) = (1.0, 1.0, 1.0, 1.0)//default is white
6.         _MaskTexture("Mask Texture", 2D) = "" {}
7.         _DiffuseTexture("Diffuse Texture", 2D) = "" {}
8.         _OverlayTexture("Overlay Texture", 2D) = "" {}
9.      }
10.     SubShader
11.     {
12.         Tags
13.         {
14.             "Queue" = "Transparent"//render after opaque layers
15.         }
16.         Pass//first pass: mask layer
17.         {
18.             Blend SrcAlpha OneMinusSrcAlpha//alpha blending
19.             Color[_MaskColour]//set 'primary' colour
20.             SetTexture[_MaskTexture]
21.             {
22.                 Combine texture * primary//apply texture * 'primary' colour
23.             }
24.         }
25.         Pass//second pass: diffuse layer
26.         {
27.             Blend SrcAlpha OneMinusSrcAlpha//alpha blending
28.             SetTexture[_DiffuseTexture]
29.             {
30.                 Combine texture//apply texture
31.             }
32.         }
33.         Pass//third pass: overlay layer
34.         {
35.             Blend SrcAlpha OneMinusSrcAlpha//alpha blending
36.             SetTexture[_OverlayTexture]
37.             {
38.                 Combine texture//apply texture
39.             }
40.         }
41.     }
42.     Fallback "Transparent/Diffuse"//if all else fails fallback to this shader
43. }

The shader takes 3 alpha transparent capable images and outputs them one after the other to produce the following:



The mask layer can then be coloured:



There are two problems I wish to resolve.

1. If an image is not supplied to the overlay layer the output is darkened and semi transparent. To resolve this I currently supply a fully transparent image to the overlay layer as the layer is not always used, think of it as a decal layer.



2. When an object using this shader is behind another object using this shader the foreground object doesn't calculate the transparency as I'd like, hiding the background object essentially.



I know the solution will be cutting the shader over to CGPROGRAM use, I just need a little help doing so.

 

Code (CSharp):

1. ZWrite Off

Solves issue #2.

Any help converting the shader to use CGPROGRAM and resolving issue #1