I have addins with both version 10.1 and 10.3 in the config.xml, so I don't think that is an issue.
I did notice though that your id= values don't match the "class" names in the original <project>_addin.py file
I think ButtonClass1 is the default, and in your _addin.py file you show map_to_kml You may have more than that that isn't showing. I have those two items batch in my addins.
Also, just fyi, most have mine have been for Catalog, not ArcMap, so there may be things I haven't run into.
Try to see if making thing match up helps.