So for 1 - it's just the 58 unique ID's in the dataset right? It doesn't sound like Date has anything to do with the view since it is the number of unique Response ID's over the entire period of time.
countd([Response ID]) on the Rows or Columns shelf provides a bar chart showing this value.
And for #2 - it's just the use of a date range filter? If so, does a date range filter on the view supply the desired outcome? I'm not sure of the need for a parameter.
If you need a parameter, to select the view, you might just use the sheet swapping technique here - http://onlinehelp.tableau.com/current/pro/desktop/en-us/help.htm#dashboards_sheet_selector.html
Two parameters with nested calculations would also work if you want to go that route.