How can I access the past weekly summaries?
I think you have missed it for last week - only works for the Monday.
When you get the weekly notification on a Monday morning - just open it up and take a pic of the totals.
or
Open Activity App and Force Press the three circles (on a Monday only) to get last weeks totals - press Weekly Summary
Doing this any other time after Monday will only give you the up-to-date totals for this week.
Hope this helps.
Ben