It would be great if the XML files distributed with the SDK and with the sim itself and its packages, and Marketplace packages, would be automatically passed through some XML validator that would not only check XML syntax but also validate against the relevant schema. The use of # means they are not even lexically valid XML, but that can be handled by a minor preprocessing step before doing the XML validation.
(In my in-progress, or more like lost-interest, experimental “template-expand.py” tool at GitHub - tml1024/MSFS-tools I replace each #FOO# with __HASH__FOO__HSAH__ to make it lexically valid XML. Even after that step some XML files from the sim would not load into Python’s xml.etree.ElementTree because of minor semantic errors that needed fixing. Like end tags not matching start tags. But it seems that many or maybe all of those have been fixed.)