@1. The preview size you've propose could be good.
@2. The About button will remain GREEN for my public versions.
Some people could receive a customized version.
@3. Could be a solution but it is away too wide for other languages.
A compromise could be to replace Edges:Slope (Kanten: Neigung) text with only Edges (Kanten).
Another solution could be to replace JList with JTable or Jeks and include a header.
The invisibility checkbox could be also added for every edge to replace the eye indicator.
@4. This setting has not the expected effect.

it is a hole file GenerateRoofPluginView.java

You've modified few lines: 177, 231, 344(// Need cast int), 346,
421, 422 (//Wrong), 478.
For the above lines was enough to make proposal like in patches.
For the createUI method it's Ok to post it entirely as you've done in the paste.

I hope to not upset you. I'm not a programmer so my opinion don't count too much.