Finding a specific XmlNode inside XmlDocument.OuterText

Question

Hi, I have the following program :

        string oldText= "<xml><xml1>text</xml1><xml1>text</xml1><xml>";
        xmlDocument.LoadXml(oldText);
        //...
        var nodes = xmlDocument.SelectNodes("//*[contains(text(), 'text')]");
        var node0 = nodes[0];
        var node1 = nodes[1];
        var newText = xmlDocument.OuterXml;

I need to find the specific index location of node0 in oldText (theoretically index = oldText.IndexOf(node0))

1. will node0 always be the first one who appears in the OuterText ?
2. how can I guarantee that oldText and newText will be exactly the same assuming that no changes were done to the values of the XML ?
3. What is the best way of finding it in C# ?

The reason for this request is : oldText comes from a text editor that supports TextMarker. I attached a marker on each node and I need to retrieve the specific marker for each node. for that I need it's location in the string.

Thanks