I had actually ordered and received a Sigma screen and the mounting kit before I got my TWD/LE. Just like you did, I ran into the same problems and was thinking WTF. In the end I carefully pulled the black plate slightly backwards in order to fit the middle standoff. This is definitely not a perfect match in any way and I take no responsibility if you try the same action and break anything. I can take some pictures and upload them tomorrow if you're interested. I'm really very surprised that none of the standoffs can be (easily) removed - what's up with that?
However, it looks great with Sigma in this machine - even if I'm a bit disappointed with the brightness of all the ColorDMD screens.