The links to the XML files are different than the links to the file in Forms Services. Go open a form in the browser and notice the URL. That is the URL you have to dynamically generate in your data view, not just the direct link to the XML file, which will always open in the rich client unless the user doesn't have IP07 (in this case, it does default to the browser).
Now, what do you mean when you say dataview and dataview web parts? I don't know of a difference. You said dataview webparts work but not "dataview." I only know of one "dataview" entity in SPD, and that's the DVWP that you put on a page to show data in a formatted manner from one or many data sources.