optimising shaderlab code and using properties variable

i've written a shader using shaderlab, that has a solid color on top of a texture and as the alpha for the solid color gets lower, the color and the texture blend together, leaving only the texture when alpha is 0. Here is the code:

  Shader "Custom/colorToOpaqueShader" {
         Properties {
             _MainTex ("main texture", 2D) = "white" {}
             _Color ("Color",Color) = (1,1,0,1)
             _ColorOpacity("Color Opacity",Float) = 1
                 }
         
         SubShader {
             
             
             Pass 
             {
             SetTexture[_MainTex]
             }
             
             Pass
             {
                 Blend SrcAlpha OneMinusSrcAlpha
                 Material 
                 {
                    Diffuse (1,1,1,.2)
                  }
              Lighting On
             }
             
             Pass
             {
             Blend SrcAlpha OneMinusSrcAlpha
           Material {
                    Diffuse [_Color]
                }
               Lighting On
               
             }
             
     
         } 
         
     }

First pass sets the texture, second pass adds a diffuse to the texture and the third pass sets out the color on top.

First question:

i would like to be able to control the color opacity using only a variable, not setting the entire color.

i.e. in the third pass i would like to have something like this:

 Diffuse(_Color.r,_Color.g,_Color.b,_ColorOpacity)

But 1) i can't get _Color.r using this sintax and 2) Although i have _ColorOpacity declared in properties, it doesn't seem to be usable (can't be recognized by unity)

Second question:

I haven't ever written a shader until today and was wondering how can i optimise this code.. 3 passes shoulds like a lot for such a small effect, i'm sure it could have been written in 1.

Third question:

How can i declare iterators in Shaderlab? i would like to have a int file that grows every frame, and have the opacity set something like

 Diffuse (.2,.2,.2, (i%40)/40)

I know i can set a variable from scripts, but i would like this functionality to be incorporated directly in a shader.

Note: If it is at your disposal, including a CG code example of this functionality would be great. I would like to start learning both shaderlab and cg to have a better grasp of the concepts