You want to use the "snapshot" method of measuring but economists use the thru-time method. The Thru-time method is the method that the US has always used with immigrants.
(2) Please answer my question as to why we should favor poorly educated illegals, who are so costly for the taxpayer, when we might substitute better educated legal immigrants who will benefit the taxpayer (according to the not-to- be- impugned NRS study)? This is true whether we look at a single generation or several.