Custom sprite shader with both greyscale and masking

I need a sprite which I can smoothly desaturate, but which ALSO works with masks. There are numerous answers online for how to either make sprite shaders which work with masks, or for shaders which have a greyscale slider, but for some reason these seem to be mutually exclusive. Adding the lines which make masks work, breaks the greyscale.

I'm a programmer but I know basically nothing about shaders. Here's what I've made from cobbling together different answers I've found. (It's probably far too long.)

Note that the lines which "toggle" the mask/greyscale functionality are those which refer to "_StencilComp."

 Shader "Custom/GreyScaleSpriteShader"
 {
     Properties
     {
         _GrayScale("Saturation", Range(0.0, 1.0)) = 0.0
         [PerRendererData] _MainTex("Sprite Texture", 2D) = "white" {}
         _Color("Tint", Color) = (1,1,1,1)
         [MaterialToggle] PixelSnap("Pixel snap", Float) = 0
 
         [HideInInspector] _RendererColor("RendererColor", Color) = (1,1,1,1)
         [HideInInspector] _Flip("Flip", Vector) = (1,1,1,1)
         [PerRendererData] _AlphaTex("External Alpha", 2D) = "white" {}
         [PerRendererData] _EnableExternalAlpha("Enable External Alpha", Float) = 0
 
         // required for UI.Mask
         [HideInInspector] _StencilComp("Stencil Comparison", Float) = 8
         [HideInInspector] _Stencil("Stencil ID", Float) = 0
         [HideInInspector] _StencilOp("Stencil Operation", Float) = 0
         [HideInInspector] _StencilWriteMask("Stencil Write Mask", Float) = 255
         [HideInInspector] _StencilReadMask("Stencil Read Mask", Float) = 255
         [HideInInspector] _ColorMask("Color Mask", Float) = 15
     }
 
     SubShader
     {
         Tags
         {
             "Queue" = "Transparent"
             "IgnoreProjector" = "True"
             "RenderType" = "Transparent"
             "PreviewType" = "Plane"
             "CanUseSpriteAtlas" = "True"
         }
 
         Cull Off
         Lighting Off
         ZWrite Off
         Blend One OneMinusSrcAlpha
 
         // required for UI.Mask
         Stencil
         {
             Ref[_Stencil]
             Comp[_StencilComp]
             Pass[_StencilOp]
             ReadMask[_StencilReadMask]
             WriteMask[_StencilWriteMask]
         }
         ColorMask[_ColorMask]
 
         Pass
         {
             CGPROGRAM
             #pragma vertex vert
             #pragma fragment frag
             #pragma multi_compile _ PIXELSNAP_ON
             #pragma multi_compile _ GRAYSCALE_ON GRAYSCALE_OFF
             #pragma multi_compile _ ETC1_EXTERNAL_ALPHA
             #include "UnityCG.cginc"
 
             struct appdata_t
             {
                 float4 vertex   : POSITION;
                 float4 color    : COLOR;
                 float2 texcoord : TEXCOORD0;
             };
 
             struct v2f
             {
                 float4 vertex   : SV_POSITION;
                 fixed4 color : COLOR;
                 half2 texcoord  : TEXCOORD0;
             };
 
             fixed4 _Color;
             float _GrayScale;
 
             v2f vert(appdata_t IN)
             {
                 v2f OUT;
                 OUT.vertex = mul(UNITY_MATRIX_MVP, IN.vertex);
                 OUT.texcoord = IN.texcoord;
                 OUT.color = IN.color * _Color;
                 #ifdef PIXELSNAP_ON
                     OUT.vertex = UnityPixelSnap(OUT.vertex);
                 #endif
 
                 return OUT;
             }
 
             sampler2D _MainTex;
 
             fixed4 frag(v2f IN) : SV_Target
             {
                 fixed4 c = tex2D(_MainTex, IN.texcoord) * IN.color;
                 c.rgb *= c.a;
 
                 if (IN.texcoord.y > _GrayScale) 
                 {
                     fixed avg = (c.r + c.g + c.b) / 3;
                     c.rgb = avg;
                 }
 
                 return c;
             }
 
             ENDCG
         }
     }
 }