Hmm, it seem to not working for me, no idea why. No errors in comsole, everything else works fine. Only mirror doedn't. Maybe I'm doing something wrong. I should select blocks I want to mirror and just click 1, 2 or 3, right?
the placing a block bug should have been fixed, and I probably wont do another GUI option, it's not inherently easy to generate that GUI. I suggest in general to consider using the hotkeys if you misclick.