Regarding your first two points:
Short answer: you can get around these problems by adding a "none of the above" option to your questions.
Long answer: for all question types, you need to check/select/edit something in the question so that the quiz will recognize that you attempted the question and then know that it should grade it. Otherwise, as you saw, you always get no credit for the question. Because of this, a non-response isn't a valid response for a question in moodle. This helps reduce the
server load because moodle can skip grading for questions that you haven't attempted.
You shouldn't be able to add a multiple choice question to the question bank unless it's possible for the student to get 100% with some combination of answers. How did you add questions where all answers give 0%? It's possible there's a bug in one of the import formats that let you import questions like this. (Or maybe the import functions aren't intended to do this check? I'm not totally sure.)
The issue with not being to deselect radio buttons happens in all HTML forms, not just moodle. Once you have a "none of the above" option, it shouldn't be a problem if a student accidentally clicks on one of the options because they can always change their answer to some option that gives 100%, whether it's the "none of the above" one or one of the other answers.
Hope this helps,
Adriane