Should IFC4X3_ADD2 OR ‘IFC4X3’ be indicated in the FILE_SCHEMA field of the IFC file?

For IFC4.3 the schema identifier as found in the express model is updated according to new revision. This means you need to be more precise in the schema identifier in the serialized models as well. If you use the 4.3 Addendum 2 schema, then that needs to be reflected in the files you write.

The official IFC 4.3 version (that is approved by ISO) is officially called the version with indeed the schema identifier IFC4X3_ADD2.