[How to properly overlap UV's?]

I've had the same problem and the copy function doesn't do too well if there's some difference in geometry. Even if you did get them straightened you would still need to stack them and there are no snapping options in the UV Preview. I ended up using blender. Here's a link about straightening in blender: http://blender.stackexchange.com/questions/41132/how-do-i-straighten-uv-maps

There are also snapping options in it's UV tools.

Hope that helps.

[UV mapping - stack islands]

I have the same kind of problem. I'm going to blender now every time I need to snap island on top of each other.