I think you're going to have to parse the file as if it was just a text file. I would consider creating a class that loads the file one line at a time into a List<string> object, and then use linq to drill down into the list.
Your main problem is that you need to handle at least three element definition variants because users are unpredictable:
Everything on a single line:
<mykey>value</mykey>
Multi-line elements:
<mykey>
value
</mykey>
Elements with an implicit closing tag:
<mykey text="value" />
It would be an interesting programming exercise that could do that, and allow features such as adding/removing elements, moving elements in the file, commenting out elements, and merely adding comments.