Problem by problem:
1) and 2)
When you use:
using Microsoft.Office.Interop.Excel
You only need to
ApplicationClass xls = new ApplicationClass();
xls.Application.Workbooks.Add(true);
Also, note that I used xls, instead of excel, as it may give you some reference error (although it should be case sensitive)...
3)
add the following line to avoid error for the XmlException:
using System.Xml;