All questions in GMAT ToolKit are from the best test prep companies or from
GMAT Club Tests. All of those resources where tested by students over a couple of years at least. So, you can rely on them. Additional questions that you can download and add to the app may come from different resources and useful if you run out of built-in questions or want to focus on specific type of questions.
Now about math. There is no "yes/no" answer to your question. Personally for me only hard math questions helped me to get Q50 but I know others who found more useful to practice with less challenging questions. It depends on your learning style, how you work with questions, and what your target is. Try to solve and understand 50 or so hard questions. If you feel you have learned new concepts and tricks, go ahead and keep solving them. If your target is 700+, the questions are just what you need, if <700, probably less challenging questions would be more appropriate taking into account that only 2 weeks left.
Let me know if you have any other questions,
Best,
Walker