Li Fei - If you enable the "Skip Card" button, you can tap on that instead of the Next button to advance to the next card and that will avoid recording a score for it - you won't be able to repeat the card, or change the answer to correct, but at least you'll avoid lowering its score. You could also install the 2.0.1 beta and enable the "prompt for answer quality" and "show all" options in the Scoring panel of Advanced Settings - that will let you mark a card correct or incorrect on your own regardless of whether you answered it correctly.
Of course this does facilitate cheating, but since Pleco is not a degree-granting institution we don't really care whether your flashcard scores are accurate - unless your Chinese teacher decides to start administering tests using Pleco flashcards you're only cheating yourself
renovator - aren't the null boxes gone in 2.0.1? Or are you not testing that yet?