Is there a way to isolate just one mountain in an alpine shader? Simple Shape seems to pick up nearby alpine mountains with it.
Not precisely, no, it's a big field of infinite Voronoi shapes...
The Painted Shader, while a bit of a hassle, is very powerful.
Head on into a Orthographic view with a width of 17000, and find a nice mountain you like. Let the preview render finish rendering, than pause the Preview. Open you Painted Shader and enter View-Relative mode at 0.05. Now outline your mountain carefully, and than fill the outline you created. Unpause the render and let it finish again to check the mask shape. If you like it, you can turn of the painted shader and you're done.
Boom. Mountain Mask.
Mask the alpine shader directly or by surface layer. You can than use you Transform Input Shader on World Position to translate it, rotate it, etc only if the Painted Shader is in Position in Terrain/Texture.
Note: Your first paint stroke WILL NOT show in the preview, but will be there after you refresh the preview. The reason you pause though is TG will refresh the render with each stroke and interfere with tracing. Well.. unless you PC is super duper. It's also good to note the Alpine Fractal shader has offset between mountains so you'll have to deal with that too.
Here is an example project file. Note the offset in the surface layers displacement and position in terrain/texture for the painted shader.