The solution, including getting the headers, is as follows:
XDocument yourXDocument = XDocument.Load(yourXmlFilePath);
IEnumerable<Tuple<XElement, IEnumerable<XElement>>> yourSubItems =
yourXDocument.Root.Descendants()
.Where(xelem => xelem.Name == "title")
.Select(xelem => new Tuple<XElement, IEnumerable<XElement>>(xelem, xelem.Parent.Elements().Where(subelem => subelem.Name.LocalName.StartsWith("subitem")));
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