Hello @KarlijnvanA, 

If the course settings have the "Participation" set to "Term", then the dates you set in here will control the access for users to participate in the course.  My answers to your questions will assume that your courses are set to use "Term" as the "Participation".

• For question 1, yes if you set a later end date for the Teacher, then teachers will have longer to access and grade assignments.  We use this feature for our courses to give teachers more time for this exact reason.
• For question 2, yes students would be able to hand in assignments until August 15th.  The exact time would depend on what you set so be aware that you may want to set the time to 11:59 PM if you want to allow until the end of day.  We also do see this course card get removed from student's dashboards after the end date.  However, they can still access the course through past courses since we don't restrict access to past courses.

It may be good to review How do term dates, course dates, and section dates work in Canvas? to learn more about the impact of the different settings for course participation and access.