This is from our Canvas Guru and it worked. It's actually cool to be able to set separate due dates for the replies:
Canvas released an update that allows instructors to set multiple due dates (discussion points) for discussions. Ex - one due date for the original post & a separate due date for replies. When they created this feature, for some reason they also updated the Speedgrader.
If you go into Speedgrader & choose the first student in the drop-down, it will automatically pull you to their first post (usually their initial post). If you hit the Next button, it'll pull you to their next post (usually their first reply). You can keep hitting the Next button to cycle through all posts the student has made. It will keep pulling you to that point in the discussion to view that student's contribution.