Question Merging two simple grayscale shaders

Hey everyone, sorry to bother you with what's likely something very dull to most of you, but I've never worked with shaders before and I'm not quite getting it.

I need to apply a grayscale shader to an Image that's within a Mask. For this, I first got the following shader that worked exactly how I wanted.

Code (CSharp):

1. Shader "Sprites-GrayScale"
2. {
3.     Properties
4.     {
5.         [PerRendererData] _MainTex ("Sprite Texture", 2D) = "white" {}
6.         _Color ("Tint", Color) = (1,1,1,1)
7.         [MaterialToggle] PixelSnap ("Pixel snap", Float) = 0
8.         _GrayscaleAmount ("Grayscale Amount", Range (0, 1)) = 1.0
9.     }
10.     SubShader
11.     {
12.         Tags
13.         {
14.             "Queue"="Transparent"
15.             "IgnoreProjector"="True"
16.             "RenderType"="Transparent"
17.             "PreviewType"="Plane"
18.             "CanUseSpriteAtlas"="True"
19.         }
20.         Cull Off
21.         Lighting Off
22.         ZWrite Off
23.         Fog { Mode Off }
24.         Blend SrcAlpha OneMinusSrcAlpha
25.         Pass
26.         {
27.         CGPROGRAM
28.             #pragma vertex vert
29.             #pragma fragment frag
30.             #pragma multi_compile DUMMY PIXELSNAP_ON
31.             #include "UnityCG.cginc"
32.          
33.             struct appdata_t
34.             {
35.                 float4 vertex   : POSITION;
36.                 float4 color    : COLOR;
37.                 float2 texcoord : TEXCOORD0;
38.             };
39.             struct v2f
40.             {
41.                 float4 vertex   : SV_POSITION;
42.                 fixed4 color    : COLOR;
43.                 half2 texcoord  : TEXCOORD0;
44.             };
45.          
46.             fixed4 _Color;
47.             v2f vert(appdata_t IN)
48.             {
49.                 v2f OUT;
50.                 OUT.vertex = UnityObjectToClipPos(IN.vertex);
51.                 OUT.texcoord = IN.texcoord;
52.                 OUT.color = IN.color * _Color;
53.                 #ifdef PIXELSNAP_ON
54.                 OUT.vertex = UnityPixelSnap (OUT.vertex);
55.                 #endif
56.                 return OUT;
57.             }
58.             sampler2D _MainTex;
59.             uniform float _GrayscaleAmount;
60.             fixed4 frag(v2f IN) : COLOR
61.             {
62.                 half4 texcol = tex2D (_MainTex, IN.texcoord);            
63.                 texcol.rgb = lerp(texcol.rgb, dot(texcol.rgb, float3(0.3, 0.59, 0.11)), _GrayscaleAmount);
64.                 texcol = texcol * IN.color;
65.                 return texcol;
66.             }
67.         ENDCG
68.         }
69.     }
70.     Fallback "Sprites/Default"
71. }

But the if I apply this shader to my Image that's within a Mask, the Mask stops working and the full image is visible.

So after some searching, I found this shader that works perfectly with my mask, but I can't change the "_GrayscaleAmount" like I could with the first shader.

Code (CSharp):

1.  
2. Shader "Custom/UISprite" {
3.     Properties
4.     {
5.        _MainTex("Base (RGB), Alpha (A)", 2D) = "white" {}
6.  
7.     // Mask cropped portion of the support
8.     //Start
9.     _StencilComp("Stencil Comparison", Float) = 8
10.     _Stencil("Stencil ID", Float) = 0
11.     _StencilOp("Stencil Operation", Float) = 0
12.     _StencilWriteMask("Stencil Write Mask", Float) = 255
13.     _StencilReadMask("Stencil Read Mask", Float) = 255
14.     _ColorMask("Color Mask", Float) = 15
15.         //End
16.     }
17.  
18.         SubShader
19.     {
20.      LOD 200
21.      Tags
22.      {
23.       "Queue" = "Transparent"
24.       "IgnoreProjector" = "True"
25.       "RenderType" = "Transparent"
26.      }
27.         // Mask cropped portion of the support
28.    //Start
29.     Stencil
30.     {
31.       Ref[_Stencil]
32.       Comp[_StencilComp]
33.       Pass[_StencilOp]
34.       ReadMask[_StencilReadMask]
35.       WriteMask[_StencilWriteMask]
36.     }
37.     ColorMask[_ColorMask]
38.         //End
39.         Pass
40.         {
41.          Cull Off
42.          Lighting Off
43.          ZWrite Off
44.          Fog { Mode Off }
45.          ColorMask RGB
46.          AlphaTest Greater .01
47.          Blend SrcAlpha OneMinusSrcAlpha
48.          ColorMaterial AmbientAndDiffuse
49.  
50.          CGPROGRAM
51.          #pragma vertex vert
52.          #pragma fragment frag
53.          #include "UnityCG.cginc"
54.          sampler2D _MainTex;
55.          struct appdata_t
56.          {
57.           float4 vertex : POSITION;
58.           half4 color : COLOR;
59.           float2 texcoord : TEXCOORD0;
60.          };
61.          struct v2f
62.          {
63.           float4 vertex : POSITION;
64.           half4 color : COLOR;
65.           float2 texcoord : TEXCOORD0;
66.          };
67.          v2f vert(appdata_t v)
68.          {
69.           v2f o;
70.           o.vertex = UnityObjectToClipPos(v.vertex);
71.           o.color = v.color;
72.           o.texcoord = v.texcoord;
73.           return o;
74.          }
75.          half4 frag(v2f IN) : COLOR
76.          {
77.              half4 col = tex2D(_MainTex, IN.texcoord) * IN.color;
78.              float c = 0.299 * col.r + 0.587 * col.g + 0.184 * col.b;
79.              col.r = col.g = col.b = c;
80.              return col;
81.             }
82.             ENDCG
83.            }
84.     }
85. }

I tried adding the field myself but that's where I'm failing. Could someone more enlightened guide me to a solution that does both? (Respecting the mask boundaries and being adjustable through a Float)

I greatly appreciate your time and kindness.