Disclaimer: I am an Apryse employee. We have recently published an article on creating ZUGFeRD invoices with iText, which may be helpful to you. Included in the article is a link to the C# version of the Java code used in the article. This example targets the BASIC profile, but can easily be adapted to other profiles the in the current (2.3) ZUGFeRD specification.

Regarding your code, the XMP exceptions are possibly due to the namespaces not being correctly registered. As for the PDF/A conformance errors, PDF/A compliance requires its own XMP metadata, so it may be you did not include this correctly?

If you still have issues, please update the question with your complete code, and an example PDF so we can investigate further.