a.) There is a way in Cornerstone. The layout elements, Section and Row and the Gap element has a feature called Hide Based on Screen Width available when you Inspect those element. You can enable that option in different screen sizes .
b.) This is not option offered in X and could be possible through custom development or a plugin.
c.) Website optimization including mobile optimization is not a service offered here. You can check out third party services like https://wpspeedguru.com/. We do have an in-house custom development team that offer paid services, who may be able to assist. They can be contacted at email@example.com if this is of interest to you. If you have any further questions about the theme, we are more than happy to provide you with assistance on these inquiries.
Thank you for your understanding.