I assume you want the following. I'm not very good at using XML so I don't understand why you need some of these!
XmlNamespaceManager.AddNamespace - I'll look at it
XMLElement - Isn't XmlNode enough? XMLElement is an XmlNode (as is XmlDocument), what does it do different that you need it as an explicit type? Is is access to the Attributes? If so how about an AttributeCollection object?
XmlDocument.CreateElement - Why not just use CreateNode("element", ...)?
XmlDocument. DocumentElement - This is actually FindChild("element") which isn't exposed at the moment so I'll add it.
XmlNode.SelectSingleNode - Do you want the overload that uses a NamespaceManager? Is that why you need it?
XmlNode.LastChild - Can't you get this by using ChildNodes and ChildCount and an XmlNodeList? Similarly for FirstChild.
I'm not being awkward but I need someone who understands XML to refine this library. A lot of the methods and properties seem to be duplicate ways of doing things and also there is lots of inheritance going on which doesn't map too well onto the Basic4ppc library model because the many types each need their own object in the library which leads to lots of duplication of objects and methods. I'd like to keep XmlNode as the container for elements and manipulate the attributes separately rather than having a lot of duplication in an XmlElement object. I am trying to keep this library as small as possible but without losing functionality.
I don't see this with a single namespace, have you changed something else as well?