verification programs or format checking programs supposed to reference the the DTD describing the "format"?

Lets make sure we understand, a URL is a URI.

And why doesn't Microsoft have a schema similarly available for its XML format?

Oh, here is the OASIS Stuff for ODT:
  • [link||Open Documnet Schema]
  • [link||Open Document Manifest Schema]
  • [link||Open Document Strict Schema]
  • [link||For those who want the PDF]
So, I guess, you now understand what I mean (I hope)

And here is a good DTD for XML in any case:
[link||W3C's XML Schema DTD] and [link||W3C's XML Schema Datatypes]
Which is a good place to start.