1. Where can we have access to emails using the mobile app
2. Same question about changing lead or opportunity stages, We don't see them in the mobile app. How can we have the same view of stages in the mobile app as in the web page.
1) As far as I know, the mobile app is not the best option for working with emails since email app could handle this task much better. You can create a detail from the activity object to view them. If you want to send emails it will be better to use mailbox application.
2) To see or edit stages you can add "stage" field to the edit page in the mobile app. If you mean DCM panel then, unfortunately, this functionality is not supported, however, this functionality may be added in the upcoming releases