I guess another issue is related to the referral to Java 8 classes from optional code, while the project is set-up to use JDK 6 by default

Actually, I think I know what you mean. I am not sure how this manifests itself in Eclipse I have several issues with Idea as well. I don't think there is much we can do right now except adding pointers to the contributing page. See also https://youtrack.jetbrains.com/issue/IDEA-85478