For the ones who missed this info (like me, because the search on this forum didn’t help me, and neither does a title like ‘collada .xsd document’).
It appears that Google Earth COLLADA 1.4 files are non-compliant:
See “Open Source Java code for COLLADA 1.4.1”
“Also, I created a method to fix a Google Earth COLLADA XML document to make it schema-compliant”
Would be great if Sketchup gets fixed, so that the Google 3D Warehouse gets filled with the proper COLLADA content. Otherwise everyone has to write fixes for kmz-dae in their im/export.