<attDef>
/@ns has a default value of http://www.tei-c.org/ns/1.0, the TEI namespace. However, all TEI attributes are unprefixed, and therefore in no namespace (according to my reading of the spec, which says "the namespace name for an unprefixed attribute name always has no value" (http://www.w3.org/TR/REC-xml-names/#defaulting). So I believe the default namespace for TEI attributes should be an empty string.
See also this ticket from last year, which may have some bearing:
https://sourceforge.net/p/tei/bugs/393/
I think you are right. I also think that default values for attributes are in general a bad idea, and in this case positively unhelpful: if you are defining a new attribute you probably don't want to put it in the TEI namespace anyway (even if there were one for attributes which, as you note, there isn't). So I have removed the <defaultVal> in question.