Thank you for getting in touch with us and bringing this issues up. I reproduced all of the cases you've mentioned. I'm glad to say, that we already fixed the class constraint issue locally, the fix just didn't make in the last release. However, the bug will be fixed with the next version of JustDecompile.
As for the other two problems, I've logged them and we'll try to fix them in a reasonable time.
Don't hesitate to tell us about any other problems in JustDecompile you encounter.