Overview

The OP basically wants to be able to use the spreadsheet functions over the original data without having to scroll across a large number of rows and columns. His background make him think about having custom views and a easy way to change from one to another but Google Sheets doesn't have a built in feature to manage custom views.

Fortunately the fourth requirements are doable by using built-in features and Google Apps Script

Rows could be hidden by using filters but columns only could be hidden manually or by using code.

For 1, and 2 use Google Apps Script to create custom menus that apply the hide/show columns and optionally rows over the "master sheet". This will allow the user to edit the data directly over the "master sheet" and sort it.

Code example

The following code creates a menu for handling the hide/show operations of columns specified in a global variable.

var cols = [1,5,7]; //columns to be hidden/shown

function onOpen() {
  var ss = SpreadsheetApp.getActiveSpreadsheet();
  var menuEntries = [ 
    {name: "Hide", functionName: "hideCols"},
    {name: "Show", functionName: "showCols"}
    ];
  ss.addMenu("Custom Views", menuEntries);
}

function hideCols() {
  var sheet = SpreadsheetApp.getActiveSheet();
  cols.forEach( function(col) {
      sheet.hideColumns(col);
    }
  )
}

function showCols() {
  var sheet = SpreadsheetApp.getActiveSheet();
  cols.forEach( function(col) {
      sheet.showColumns(col);
    }
  )
}

Indeed, the first invocation of importrange cannot be nested within another function: the request for authorization does not "bubble up" (at first I thought this was an error on Google's part, but then considered that a single command can combine several importrange within it, so providing an authorization dialog in such a case would be extra complicated).

But you don't need a separate sheet to handle authorization. Just make the first call to the spreadsheet a single cell request: importRange("key","A1"). Then this can be replaced with the countunique that you want. Or, you can have a column with keys, and another column with a single cell request for authorization purpose.