If you want any particular sheet to be unaffected by the lighting it needs to be above the sheet with the Wall Shadow, Point Light Finalize effect on it. Then you can control how that sheet appears the normal way using colour adjusting sheet effects like Colorize, or Adjust Hue/Saturation.

When I say 'above' I mean as you are looking at the map. In the actual Sheets and Effects dialog that would be below it in the list.

You can use the lake polygon as a cut out shape on more than one sheet. I've set it up for you in this version. All I did was add a Color Key sheet effect to the TERRAIN FARMLAND sheet, change the properties of the lake to solid colour 6 (magenta), moved it to the TERRAIN FARMLAND sheet and copied it from there to the LAND sheet.

There's nothing left on the actual LAKES sheet now.

I hope this is what you wanted?

Yes. There are 2 ways you can do it. One is to make your own drawing tool so you can use the trace function, and the other is to use a Color Key sheet effect. I prefer the drawing tool method.

If you have modified or made your own drawing tool before you will already know how to do this, but if you haven't it's not too difficult to do. Open the drawing tools dialog and click the advanced button. Pick a simple tool that draws a straight polygon from the list of those available. Although your cave has fractal edges a straight drawing tool will do the best job for tracing it. Then click New and give your new tool a name. Ok and then click the Options button. That will open the Custom tool properties dialog like this one below. The settings you make here are how you make your tool do what you want it to do. So pick the right fill, layer and sheet and ok the settings. (This is just a random drawing tool for illustration purposes and won't have the right settings for your tool)

The next thing you need to do is Save your new tool. Once that is done click Ok and you should be able to use it to draw the desired shape using the trace function of the drawing tool to trace around your cave in a few seconds.

When you want the background to be above the lighting you need to move it above the lighting, or below all the lighting sheets in the sheets and effects list. It actually ceases to be a background and becomes a foreground of sorts.

To get it that way the 'background' has to be drawn as a polygon on a sheet above the rest of the map (but normally below the SCREEN and TEXT sheets). Basically it will be a polygon that covers everything that isn't part of the cave.

Just to clear up any confusion, here is the spreadsheet showing what is in each pack.