You can use  XML import and export modules to connect XML applications with databases. In some trivial cases there will be a 1:1 match between feildnames in the database table and element type names in the XML Schema or DTD, but in most cases some programming will be required to establish the desired match. This can usually be stored as a procedure so that subsequent uses are simply commands or calls with the relevant parameters.

No. This is done in the document's Document Type Declaration, not in the DTD.

No. XML namespaces can be declared only on elements and their scope consists only of those elements and their descendants. Thus, the scope can never include the DTD.

  Yes, the W3C recommends using XHTML which is 'a reformulation of HTML 4 in XML 1.0'. This specification defines HTML as an XML application, and provides three DTDs corresponding to the ones defined by HTML 4.* (Strict, Transitional, and Frameset). 

TO make existing html files to xml you can convert them to conform to some new document type (with or without a DTD or Schema) and write a stylesheet to go with them; or edit them to conform to XHTML.
It is necessary to convert existing HTML files because XML does not permit end-tag minimisation (missing , etc), unquoted attribute values, and a number of other SGML shortcuts which have been normal in most HTML DTDs.