If something goes "horrible wrong" (which is often nothing more than you forgot a "<"
) you can debug it with Notepad++. There is a special XML debug tool, but I can't tell you what it is called
You could look over your files again and make sure that:
1) every tag starts with < and ends with >
2) every tag that opens <tag> close after the imput </tag> (or is closed <tag/>)
As for searchiing the correct tag for a name, look at other units and/or buildings that gives that promo. Or (I don't know if this exists) look in the promotion file and look for a tag that might be the right one. You will have fun when looking up techs in the TH era
but if you keep on doing so, you know what tag is what name, or you can make a .txt file as dictionary.