I missed the very last part of your 1st post before:
There is a material function for this. I think its called “Manual Landscape UVs” or something like that. I made it for pretty much the same reason on the kite demo. It works by having two inputs, Min and Max Position. These are measured in the editor. safest way is to place some kind of ‘marker’ actors in your level at the min/max extents and use those when cropping the image in PS as well as for mapping the texture.
I see what you mean based on that image. The shore is far too broken up and organic to attempt a simple skirt method.