Invalid XDocument is getting validated

Question

I have following xml schema

Answer 1

The issue is that both cases 2 and 3 are valid per the schema - your schema doesn't have any opinion on elements in namespaces other than its targetNamespace.

XmlReader can return a warning for this, but there's no overload for XDocument that will do it. The snippet in your linked question uses an XmlReader wrapper around XDocument, I can't see why you'd have any issue with doing it this way.