Add a question bank (quiz) feature to a Wordpress theme

We require an e-learning custom question bank/quiz feature to be added to our Wordpress website which is based on an e-learning theme. The theme already has a basic quiz feature which can be used as a starting point, however we need to add several additional features as follows: • Quiz with time limit which can be enabled or disabled by the user. • Question categories and sub-categories within a quiz that can be enabled or disabled by the user. • Correct answer shown with a text explanation and/or an embedded (link provided) video explanation. • Retry quiz with only the questions that were answered incorrectly. • Randomize question order. • Percentage of users that selected each answer shown after. • Progress bar to show how many questions completed and remaining in the quiz, as well as a % completed. • Admin should be able to mark questions as being easy/moderate/difficult when creating them. User should see a small box which states the question difficulty when answering it in a quiz. • Attempt only easy/moderate/difficult questions. • Admin should be able to add a ‘Resources’ field for each question when creating it. User should get a ‘Resources’ box after answering a question. • Admin should be able to add a ‘Info' field for each question when creating it. User will see this info when answering the question and will provide constant values that are needed to calculate the answer. • User should have the ability to flag a question. • Question navigation - ability to move forward and back in a quiz or skip to a particular question number. Flagged questions should also show up in this navigation. All of the features must be embedded with the existing theme quiz functionality/style (a modern style would be appreciated with smooth drop downs and transitions) and we would appreciate clean code with commenting and documentation. We will provide detailed examples of the features and the behavior required. The purpose of this quiz/question bank is to prepare the user for a real exam. The user will see the correct answer after each question. These features are to improve the user’s learning experience and the quiz acts as a sort of trial/mock exam.