Logger returns "Range" because your variable is just a range. To get an array of the values in that range, you need to use the .getValues() method.
var daterange = sourcesheet.getRange("H"+source_last_row+":Q"+source_last_row);
var daterangeVals = daterange.getValues();
You can then use the .toString() method to convert the array to string.
That will return the values separated by a comma. As you want a comma and a space, you could then use the .replace() method on the string.