Personally I never allow Skip even on normal quiz slides. D&D slides are not totally the same as the default SCORM type of question slides. I have published several blogs to have them look more like question slides, but never talked about the Skip button. It has nothing to do with the style of the Submit button at all. Skip means that the learner is not taking that question. The Submit button mostly is of the type 'Transparent button'. You can add a button of the same type and use the Quiz button default style. The action for that button is simple 'Go to Next Slide' which is the default style of any new button.
What is exactly the problem? If you have a playbar, just clicking the Forward button is also leading to skipping the question. I am smiling, because once I wrote this blog, which is quite the contrary from your question:
http://blog.lilybiri.com/prevent-skipping-on-drag-and-drop-quiz-slide