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|http://www.oasis-open.org/committees/download.php/12571/OpenDocument-schema-v1.0-os.rng|Open Documnet Schema]
  • [link|http://www.oasis-open.org/committees/download.php/12570/OpenDocument-manifest-schema-v1.0-os.rng|Open Document Manifest Schema]
  • [link|http://www.oasis-open.org/committees/download.php/12569/OpenDocument-strict-schema-v1.0-os.rng|Open Document Strict Schema]
  • [link|http://www.oasis-open.org/committees/download.php/12572/OpenDocument-v1.0-os.pdf|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|http://www.w3.org/2001/XMLSchema.dtd|W3C's XML Schema DTD] and [link|http://www.w3.org/2001/XMLSchema-datatypes|W3C's XML Schema Datatypes]
Which is a good place to start.