That's odd. Is this in Unity 5 or Unity 4?

Is it still crashing when you drag a substance texture into a Texture2D node?

Before we look into this further, you should use the latest version of Shader Forge

This may come in the future, but not in the near future

That's fine! Check out the readme :)

Glad it worked out :)

There shouldn't be much of a difference at all. The SF viewport is using a material using your shader, lit by two directional lights. I would guess that you forgot to assign something in the material you use in the scene, VS the one in SF.
It could also be something that went very wrong with forward/deferred though. Not sure to be honest

Not at the moment, but it's something that might come in the future

You can do that too. Just pass in a regular texture instead of the scene content, and you have the same result. It's a bit trickier than modifying regular textures though, but, it should work. That said, it would be a bit easier with multi-pass shaders, which isn't possible in SF at the moment.