Constructor Problems and Questions

When making a roof I encountered a number of problems. First of all I made a segment of the roof and wanted to rotate it, so I set the "action center" to "group center" and hit "reset selected pivot points". The pivot point was not in the center of the group, and in fact was too far + on the Z and Y axis. This set of the next problem.

Now I had two segments of roof that were offset and had to zoom in really really close to freehand adjust the pieces together. Using snap would never allow the two groups to line up, no matter what I set the grid to. So now I have the pieces lined up decently, so (inside constructor) they appear next to each other.

Exporting this as a .DIF makes spaces between my roof pieces! Also, one of my roof segments was simply missing from the interior. Firstly, is there a way to "snap" two faces from separate brushes together, so they can be attached perfectly by Constructor without the error of a human doing it by hand? Secondly, does anyone know why my roof segment disappeared (it was exactly like the other roof segments)?

I thank any of you who reply with any tips, advice or answers.

Edit: It appears that the brushes aren't actually missing, just that only one side of the brush is rendering, where the other side doesn't show up...so it's like looking at a hole through the roof on the outside but on the inside you can see the texture.